Ecosyste.ms: Awesome
An open API service indexing awesome lists of open source software.
https://github.com/sudheerj/ECMAScript-features
ECMAScript features cheatsheet
https://github.com/sudheerj/ECMAScript-features
ecmascript ecmascript-features ecmascript6 es2015 es2016 es2017 es2018 es2019 es2020 es2020-new-features es2021 es2022 es2023 es6 javascript javascript-es6
Last synced: 27 days ago
JSON representation
ECMAScript features cheatsheet
- Host: GitHub
- URL: https://github.com/sudheerj/ECMAScript-features
- Owner: sudheerj
- Created: 2020-04-25T10:45:02.000Z (about 4 years ago)
- Default Branch: master
- Last Pushed: 2024-02-20T23:28:46.000Z (5 months ago)
- Last Synced: 2024-04-15T04:34:40.727Z (3 months ago)
- Topics: ecmascript, ecmascript-features, ecmascript6, es2015, es2016, es2017, es2018, es2019, es2020, es2020-new-features, es2021, es2022, es2023, es6, javascript, javascript-es6
- Language: JavaScript
- Homepage:
- Size: 201 KB
- Stars: 701
- Watchers: 23
- Forks: 225
- Open Issues: 0
-
Metadata Files:
- Readme: README.md
Lists
- awesome-random-stuff - ECMAScript Features or Cheatsheet
README
# ECMAScript Features or Cheatsheet
> Click :star:if you like the project. Pull Request are highly appreciated. Follow me [@SudheerJonna](https://twitter.com/SudheerJonna) for technical updates.
## How to run examples
```cmd
npm install
npx babel-node es2020/bigint // Try other examples too
```
# What is ECMAScript?
**ECMAScript** is the scripting language which acts as the basis of JavaScript. ECMAScript standardized by the ECMA International standards organization in the ECMA-262 and ECMA-402 specifications.
Each proposal for an ECMAScript feature goes through the following maturity stages:
1. Stage 0: Strawman;
2. Stage 1: Proposal;
3. Stage 2: Draft;
4. Stage 3: Candidate;
5. Stage 4: Finished.
## Version History
| Edition | Date |
| --- | --------- |
| ES2015 Or ES6 | June 2015 |
| ES2016 Or ES7 | June 2016 |
| ES2017 Or ES8 | June 2017 |
| ES2018 Or ES9 | June 2018 |
| ES2019 Or ES10 | June 2019 |
| ES2020 Or ES11 | June 2020 |
| ES2021 Or ES12 | June 2021 |
| ES2022 Or ES13 | June 2022 |
| ES2023 Or ES14 | June 2023 |
### Table of Contents
| No. | Feature |
| --- | --------- |
| | **ES2015 Or ES6** |
|1 | [Variable Scoping](#variable-scoping) |
|2 | [Arrow functions](#arrow-functions) |
|3 | [Classes](#classes) |
|4 | [Enhanced object literals](#Enhanced-object-literals) |
|5 | [Template literals](#template-literals) |
|6 | [Destructuring](#destructuring) |
|7 | [Default parameters](#default-parameters) |
|8 | [Rest parameter](#rest-parameter) |
|9 | [Spread Operator](#spread-operator) |
|10 | [Iterators & For..of](#iterators-&-forof) |
|11 | [Generators](#generators) |
|12 | [Modules](#modules) |
|13 | [Set](#set) |
|14 | [Weakset](#weakset) |
|15 | [Map](#map) |
|16 | [Weakmap](#weakmap) |
|17 | [Unicode](#unicode) |
|18 | [Proxies](#proxies) |
|19 | [Symbols](#symbols) |
|20 | [Promises](#promises) |
|21 | [Reflect](#reflect) |
|22 | [Binary and Octal](#binary-and-octal) |
|23 | [Proper Tail calls](#proper-tail-calls)|
|24 | [Array find methods](#array-find-methods)
| | **ES2016 Or ES7** |
|1 | [Array includes](#array-includes) |
|2 | [Exponentiation Operator](#exponentiation-operator) |
| | **ES2017 Or ES8** |
|1 | [Async functions](#async-functions) |
|2 | [Object values](#object-values) |
|3 | [Object entries](#object-entries) |
|4 | [Object property descriptors](#object-property-descriptors) |
|5 | [String padding](#string-padding)|
|6 | [Shared memory and atomics](#shared-memory-and-atomics)|
|7 | [Trailing commas](#trailing-commas) |
| | **ES2018 Or ES9**|
|1 | [Async iterators](#async-iterators) |
|2 | [Object rest and spread operators](#object-rest-and-spread-operators) |
|3 | [Promise finally](#promise-finally) |
| | **ES2019 Or ES10**|
|1 | [Array flat and flatMap](#array-flat-and-flatmap) |
|2 | [Object formEntries](#object-formentries) |
|3 | [String trimStart and trimEnd](#string-trimstart-and-trimend) |
|4 | [Symbol description](#symbol-description)|
|5 | [Optional Catch Binding](#optional-catch-binding)|
|6 | [JSON Improvements](#json-improvements)|
|7 | [Array Stable Sort](#array-stable-sort)|
|8 | [Function.toString()](#functiontostring())|
|9 | [Private Class Variables](#private-class-variables)|
| | **ES2020 Or ES11**|
|1 | [BigInt](#bigint) |
|2 | [Dynamic Import](#dynamic-import) |
|3 | [Nullish Coalescing Operator](#nullish-coalescing-operator)|
|4 | [Optional chaining](#optional-chaining)|
|5 | [Promise allSettled](#promiseallsettled)|
|6 | [String matchAll](#string-matchall)|
|7 | [globalThis](#globalthis)|
|8 | [import.meta](#importmeta)|
|9 | [for..in order](#forin-order)|
| | **ES2021 Or ES12**|
|1 | [replaceAll](#replaceall) |
|2 | [promise.any](#promiseany) |
|3 | [WeakRef](#weakref)|
|4 | [Numeric Separators](#numeric-separators)|
|5 | [Logical Operators](#logical-operators)|
| | **ES2022 Or ES13**|
|1 | [Top level await](#top-level-await) |
|2 | [Class fields](#class-fields) |
|3 | [Array at()](#array-at())|
|4 | [Error cause](#error-cause)|
|5 | [hasOwn](#has-own)|
|6 | [Regex match indices](#regex-match-indices)|
| | **ES2023 Or ES14**|
|1 | [Find array from last](#find-array-from-last) |
|2 | [Hashbang syntax](#hashbang-syntax) |
|3 | [Symbols as WeakMap keys](#symbols-as-weakmap-keys)|
|4 | [Change Array by Copy](#change-array-by-copy)|
## ES2015 Or ES6
1. ### Variable Scoping
The variable scoping determines the visibility or accessibility of a variable within the certain part of the program or region.
In ES6, both `const` and `let` keywords allow developers to declare variables in the block scope.
The `let` statement declares a block-scoped local variable which can be reassigned. i.e, `let` declaration creates a mutable variable.
```js
let a = 1;
if (a === 1) {
let a = 2;
console.log(a); //2
}
console.log(a); //1
```
**const** variables are similar to **let** variables but they can't be changed through reassignment. i.e, The const declaration creates a read-only reference to a value.
```js
const x = 1;
if (x === 1) {
const y = 2; // You cannot re-assign the value similar to let variable
console.log(y); //2
}
console.log(x); //1
```
**[⬆ Back to Top](#table-of-contents)**
2. ### Arrow functions
The arrow functions provides a more concise syntax for writing function expressions by opting out the function and return keywords using fat arrow(=>) notation. Let's see how this arrow function looks like,
```js
// Function Expression
var multiplyFunc = function(a, b) {
return a * b;
}
console.log(multiplyFunc(2, 5)); // 10
// Arrow function
var multiplyArrowFunc = (a, b) => a * b;
console.log(multiplyArrowFunc(2, 5)); // 10
```
You can also skip parenthesis(()) if the function has exactly one parameter(either zero or more than one parameter). Apart from this, you can wrap braces({}) if the function has more than one expression in the body.
Let's list down all the variations of arrow functions,
```js
//1. Single parameter and single statement
const message = name => console.log("Hello, " + name + "!");
message("Sudheer"); // Hello, Sudheer!
//2. Multiple parameters and single statement
const multiply = (x, y) => x * y;
console.log(multiply(2, 5)); // 10
//3. Single parameter and multiple statements
const even = number => {
if(number%2) {
console.log("Even");
} else {
console.log("Odd");
}
}
even(5); // odd
//4. Multiple parameters and multiple statements
const divide = (x, y) => {
if(y != 0) {
return x / y;
}
}
console.log(divide(100, 5)); // 20
//5. No parameter and single statement
const greet = () => console.log('Hello World!');
greet(); // Hello World!
```
**[⬆ Back to Top](#table-of-contents)**
3. ### Classes
The classes are introduced as syntactic sugar over existing prototype based inheritance and constructor functions. So this feature doesn't bring new object-oriented inheritance model to JavaScript.
There are two ways to define classes,
1. **Class declarations:**
```js
class Square {
constructor(length) {
this.length = length;
}
get area() {
return this.length * this.length;
}
set area(value) {
this.area = value;
}
}
```
2. **Class expressions:**
```js
const square = class Square {
constructor(length) {
this.length = length;
}
get area() {
return this.length * this.length;
}
set area(value) {
this.area = value;
}
}
```
You can use **extend** keyword to use inheritance. This enables the subclass to get all features of a parent class.
```js
class Vehicle {
constructor(name) {
this.name = name;
}
start() {
console.log(`${this.name} vehicle started`);
}
}
class Car extends Vehicle {
start() {
console.log(`${this.name} car started`);
}
}
const car = new Car('BMW');
console.log(car.start()); // BMW car started
```
**Note:** Even though ES6 classes looks similar to classes in other object oriented languages, such as Java, PHP, etc but they do not work exactly the same way.
**[⬆ Back to Top](#table-of-contents)**
4. ### Enhanced object literals
Object literals are extended to support setting the prototype at construction, shorthand for foo: foo assignments, defining methods, making super calls, and computing property names with expressions.
The important enhancements of object literals are,
1. **Property Shorthand:**
Object's properties are often created from variables with the same name.
Let's see the ES5 representation
```js
var a = 1, b = 2, c = 3,
obj = {
a: a,
b: b,
c: c
};
console.log(obj);
```
and it can be represented in a shorter syntax as below,
```js
const a = 1, b = 2, c = 3;
const obj = {
a,
b,
c
};
console.log(obj);
```
2. **Method Shorthand:**
In ES5, Object methods require the function statement as below,
```js
var calculation = {
sum: function(a, b) { return a + b; },
multiply: function(a, b) { return a * b; }
};
console.log(calculation.sum(5, 3)); // 8
console.log(calculation.multiply(5, 3)); // 15
```
This can be avoided in ES6,
```js
const calculation = {
sum(a, b) { return a + b; },
multiply(a, b) { return a * b; }
};
console.log(calculation.sum(5, 3)); // 8
console.log(calculation.multiply(5, 3)); // 15
```
3. **Computed Property Names:**
In ES5, it wasn’t possible to use a variable for a key name during object creation stage.
```js
var
key = 'three',
obj = {
one: 1,
two: 2
};
obj[key] = 3;
```
Object keys can be dynamically assigned in ES6 by placing an expression in square brackets([])
```js
const
key = 'three',
computedObj = {
one: 1,
two: 2,
[key]: 3
};
```
**[⬆ Back to Top](#table-of-contents)**
5. ### Template literals
Prior to ES6, JavaScript developers would need to do ugly string concatenation to creat dynamic strings.
Template literals allows you to work with strings in a new way compared to ES5. These are just string literals allowing embedded expressions denoted by the dollar sign and curly braces (`${expression}`). Also, these literals are enclosed by the backtick (`` ` ``) character instead of double or single quotes.
ES6 has two new kinds of literals:
1. **Template literals:** string literals which exists across multiple lines and include interpolated expressions(i.e, ${expression})
```js
const firstName = 'John';
console.log(`Hello ${firstName}!
Good morning!`);
```
2. **Tagged template literals:** Function calls which are created by mentioning a function before a template literal.
The real world use case is creating components in CSS-In-JS styled components to use across the application
```js
const Button = styled.a`
display: inline-block;
border-radius: 3px;
`
```
**[⬆ Back to Top](#table-of-contents)**
6. ### Destructuring
Destructuring is a javascript expression for extracting multiple values from data stored in objects(properties of an object) and Arrays.
**Object destructuring:**
This feature is used to extract values from an object.
```js
const user = { firstName: 'John', lastName: 'Kary' };
const {firstName, lastName} = user;
console.log(firstName, lastName); // John, Kary
```
**Array destructuring:**
This feature is used to extract values from an array.
```js
const [one, two, three] = ['one', 'two', 'three'];
console.log(one, two, three); // one, two, three
```
You can use destructing in below places,
1. Variable declarations
2. Assignments
3. Parameter definitions
4. for-of loop
**[⬆ Back to Top](#table-of-contents)**
7. ### Default parameters
Default parameters allow named parameters of a function to be initialized with default values if no value or undefined is passed.
Prior to ES6, you need check for undefined values and provide the default value for undefined values using if/else or ternary operator
```js
function add(a, b) {
a = (typeof a !== 'undefined') ? a : 10;
b = (typeof b !== 'undefined') ? b : 20;
return a + b;
}
add(20); // 40
add(); // 30
```
In ES6, these checks can be avoided using default parameters
```js
function add(a = 10, b = 20) {
return a + b;
}
add(20); // 40
add(); // 30
```
**[⬆ Back to Top](#table-of-contents)**
8. ### Rest parameter
The rest parameter is used to represent an indefinite number of arguments as an array. The important point here is only the function's last parameter can be a "rest parameter". This feature has been introduced to reduce the boilerplate code that was induced by the arguments.
```js
function sum(...args) {
return args.reduce((previous, current) => {
return previous + current;
});
}
console.log(sum(1, 2, 3)); // 6
console.log(sum(1, 2, 3, 4)); // 10
console.log(sum(1, 2, 3, 4, 5)); // 15
```
**[⬆ Back to Top](#table-of-contents)**
9. ### Spread Operator
Spread Operator allows iterables( arrays / objects / strings ) to be expanded into single arguments/elements.
1. In function and constructor calls, the spread operator turns iterable values into arguments
```js
console.log(Math.max(...[-10, 30, 10, 20])); //30
console.log(Math.max(-10, ...[-50, 10], 30)); //30
```
2. In Array literals and strings, the spread operator turns iterable values into Array elements
```js
console.log([1, ...[2,3], 4, ...[5, 6, 7]]); // 1, 2, 3, 4, 5, 6, 7
```
**Note:** The spread syntax is opposite of rest parameter.
**[⬆ Back to Top](#table-of-contents)**
10. ### Iterators & For..of
String, Array, TypedArray, Map, and Set are all built-in iterables but objects are not iterables by default.
Iterators are a new way to loop over any collection in JavaScript. These are objects which defines a sequence and potentially a return value upon its termination.
An iterator implements the Iterator protocol by having a next() method that returns an object with two properties:
1. **value:** The next value in the iteration sequence.
2. **done:** returns true if the last value in the sequence has already been consumed.
You can make the object iterable by defining a `Symbol.iterator` property on it.
```js
const collection = {
one: 1,
two: 2,
three: 3,
[Symbol.iterator]() {
const values = Object.keys(this);
let i = 0;
return {
next: () => {
return {
value: this[values[i++]],
done: i > values.length
}
}
};
}
};
const iterator = collection[Symbol.iterator]();
console.log(iterator.next()); // → {value: 1, done: false}
console.log(iterator.next()); // → {value: 2, done: false}
console.log(iterator.next()); // → {value: 3, done: false}
console.log(iterator.next()); // → {value: undefined, done: true}
for (const value of collection) {
console.log(value);
}
```
The for...of statement creates a loop iterating over user defined collection object. But this loop can be used for built-in objects too.
**Note:** The abrupt iteration termination can be caused by break, throw or return.
**[⬆ Back to Top](#table-of-contents)**
11. ### Generators
A generator is a function that can stop or suspend midway and then continue from where it stopped while maintaining the context(saved across re-entrances). It can be defined using a function keyword followed by an asterisk(i.e, function* ()).
This function returns an iterator object and this iterator's **next()** method returns an object with a value property containing the yielded value and a done property which indicates whether the generator has yielded its last value.
```js
function* myGenerator(i) {
yield i + 10;
yield i + 20;
return i + 30;
}
const myGenObj = myGenerator(10);
console.log(myGenObj.next().value); // 20
console.log(myGenObj.next().value); // 30
console.log(myGenObj.next().value); // 40
```
**Note:** We can use `yield*` to delegate to another generator function
**[⬆ Back to Top](#table-of-contents)**
12. ### Modules
Modules are small units of independent, reusable code to be used as the building blocks in a Javascript application.
Prior to ES6, there was no native modules support in JavaScript. There were 3 major module standards used,
1. Asynchronous Module Definition (AMD)
2. RequireJS Modules
3. CommonJS Modules (module.exports and require syntax used in Node.js)
ES6 has provided the built-in support for modules. Everything inside a module is private by default, and runs in strict mode. Public variables, functions and classes are exposed using `export` statement and import the same using `import` statement.
**Export Statement:**
There are two types of exports:
1. Named Exports (Zero or more exports per module)
You can export each element or a single export statement to export all the elements at once
```js
// module "my-module.js"
const PI = Math.PI;
function add(...args) {
return args.reduce((num, tot) => tot + num);
}
function multiply(...args) {
return args.reduce((num, tot) => tot * num);
}
// private function
function print(msg) {
console.log(msg);
}
export { PI, add, multiply };
```
2. Default Exports (One per module)
If we want to export a single value, you could use a default export
```js
// module "my-module.js"
export default function add(...args) {
return args.reduce((num, tot) => tot + num);
}
```
**Import Statement:**
The static import statement is used to import read only live bindings which are exported by another module.
There are many variations of import scenarios as below,
```js
// 1. Import an entire module's contents
import * as name from "my-module";
//2.Import a single export from a module
import { export1 } from "my-module";
//3.Import multiple exports from a module
import { export1 , export2 } from "my-module";
//4.Import default export from a module
import defaultExport from "my-module";
//5.Import an export with an alias
import { export1 as alias1 } from "my-module";
```
**[⬆ Back to Top](#table-of-contents)**
13. ### Set
Set is a built-in object to store collections of unique values of any type.
```js
let mySet = new Set()
mySet.add(1);
mySet.add(2);
mySet.add(2);
mySet.add('some text here');
mySet.add({one: 1, two: 2 , three: 3});
console.log(mySet); // Set [ 1, 2, 'some text here', {one: 1, two: 2 , three: 3} ]
console.log(mySet.size) // 4
console.log(mySet.has(2)); // true
```
**[⬆ Back to Top](#table-of-contents)**
14. ### Weakset
The Set is used to store any type of data such as primitives and object types. Whereas WeakSet is an object to store weakly held objects in a collection. (i.e, WeakSet is the collections of objects only). Here weak means, If no other references to an object stored in the WeakSet exist, those objects can be garbage collected.
```js
let myUserSet = new WeakSet();
let john = { name: "John" };
let rocky = { name: "Rocky" };
let alex = { name: "Alex" };
let nick = { name: "Nick" };
myUserSet.add(john);
myUserSet.add(rocky);
myUserSet.add(john);
myUserSet.add(nick);
console.log(myUserSet.has(john)); // true
console.log(myUserSet.has(alex)); // false
console.log(myUserSet.delete(nick));
console.log(myUserSet.has(nick)); // false
john = null;
```
**[⬆ Back to Top](#table-of-contents)**
15. ### Map
Map is a collection of elements where each element is stored as a Key, value pair. It can hold both objects and primitive values as either key or value and iterates its elements in insertion order.
Let's take a map with different types of primitives and objects as key-value pairs and various methods on it,
```js
let typeMap = new Map();
var keyObj = {'one': 1}
typeMap.set('10', 'string'); // a string key
typeMap.set(10, 'number'); // a numeric key
typeMap.set(true, 'boolean'); // a boolean key
typeMap.set(keyObj, 'object'); // an object key
console.log(typeMap.get(10)); // number
console.log(typeMap.get('10')); // string
console.log(typeMap.get(keyObj)); // object
console.log(typeMap.get({'one': 1})); // undefined
console.log(typeMap.size ); // 4
for(let item of typeMap) {
console.log(item);
}
for(let item in typeMap) {
console.log(item);
}
```
**[⬆ Back to Top](#table-of-contents)**
16. ### Weakmap
WeakMap object is a collection of key/value pairs in which the keys are weakly referenced. For this object, the keys must be objects and the values can be arbitrary values.
Let's see various methods of weakmap with below example,
```js
var weakMap = new WeakMap();
var obj1 = {}
var obj2 = {}
weakMap.set(obj1, 1);
weakMap.set(obj2, 2);
weakMap.set({}, {"four": 4});
console.log(weakMap.get(obj2)); // 2
console.log(weakMap.has({})); // return false even though empty object exists as key. Because the keys have different references
delete obj2;
console.log(weakMap.get(obj2)); // 2
weakMap.delete(obj1)
console.log(weakMap.get(obj1)); //undefined
```
**[⬆ Back to Top](#table-of-contents)**
17. ### Unicode
Prior to ES6, JavaScript strings are represented by 16-bit character encoding (UTF-16). Each character is represented by 16-bit sequence known as code unit. Since the character set is been expanded by Unicode, you will get unexpected results from UTF-16 encoded strings containing surrogate pairs(i.e, Since it is not sufficient to represent certain characters in just 16-bits, you need two 16-bit code units).
```js
let str = '𠮷';
console.log(str.length); // 2
console.log(text.charAt(0)); // ""
console.log(text.charAt(1)); // ""
console.log(text.charCodeAt(0)); // 55362(1st code unit)
console.log(text.charCodeAt(1)); // 57271(2nd code unit)
console.log(/^.$/.test(str)); // false, because length is 2
console.log('\u20BB7'); // 7!(wrong value)
console.log(str === '\uD842\uDFB7'); // true
```
ECMAScript 6 added full support for UTF-16 within strings and regular expressions. It introduces new Unicode literal form in strings and new RegExp u mode to handle code points, as well as new APIs(codePointAt, fromCodePoint) to process strings.
```js
let str = '𠮷';
// new string form
console.log('\u{20BB7}'); // "𠮷"
// new RegExp u mode
console.log(new RegExp('\u{20BB7}', 'u'));
console.log(/^.$/u.test(str)); // true
//API methods
console.log(str.codePointAt(0)); // 134071
console.log(str.codePointAt(1)); // 57271
console.log(String.fromCodePoint(134071)); // "𠮷"
```
**[⬆ Back to Top](#table-of-contents)**
18. ### Symbols
Symbol is a new peculiar primitive data type of JavaScript, along with other primitive types such as string, number, boolean, null and undefined. The new symbol is created just by calling the Symbol function. i.e, Every time you call the Symbol function, you’ll get a new and completely unique value. You can also pass a parameter to Symbol(), which is useful for debugging purpose only.
Even though equality checks on two symbols is always false, it will be true while comparing symbols with `.for` method due to global registry (i.e, Symbols.for('key') === Symbols.for('key'))
These symbols are useful to uniquely identify properties or unique constants,
```js
//1. Object properties
let id = Symbol("id");
let user = {
name: "John",
age: 40,
[id]: 111
};
for (let key in user) {
console.log(key); // name, age without symbols
}
console.log(JSON.stringify(user)); // {"name":"John", "age": 40}
console.log(Object.keys(user)); // ["name", "age"]
console.log( "User Id: " + user[id] ); // Direct access by the symbol works
//2. Unique constants
const logLevels = {
DEBUG: Symbol('debug'),
INFO: Symbol('info'),
WARN: Symbol('warn'),
ERROR: Symbol('error'),
};
console.log(logLevels.DEBUG, 'debug message');
console.log(logLevels.INFO, 'info message');
//3. Equality Checks
console.log(Symbol('foo') === Symbol('foo')); // false
console.log(Symbol.for('foo') === Symbol.for('foo')); // true
```
**[⬆ Back to Top](#table-of-contents)**
19. ### Proxies
The Proxy object is used to create a proxy for another object, which can intercept and redefine fundamental operations for that object such as property lookup, assignment, enumeration, function invocation etc. These are used in many libraries and some browser frameworks.
The proxy object is created with two parameters with below syntax,
```js
let proxy = new Proxy(target, handler)
```
1. **target:** Object on which you want to proxy
2. **handler:** An object that defines which operations will be intercepted and how to redefine them.
The property Lookup Behavior of a user proxied object will be as below,
```js
const target = {
name: "John",
age: 3
};
const handler = {
get: function(target, prop) {
return prop in target ?
target[prop] :
`${prop} does not exist`;
}
};
const user = new Proxy(target, handler);
console.log(user.name); // John
console.log(user.age); // 3
console.log(user.gender); // gender does not exist
```
These proxies also enforce value validations. Let's take an example with set handler,
```js
let ageValidator = {
set: function(obj, prop, value) {
if (prop === 'age') {
if (!Number.isInteger(value)) {
throw new TypeError('The age is not an integer');
}
if (value > 200) {
throw new RangeError('Invalid age');
}
}
obj[prop] = value; // The default behavior to store the value
return true; // Indicate success
}
};
const person = new Proxy({}, ageValidator);
person.age = 30;
console.log(person.age); // 30
person.age = 'old'; // Throws an exception
person.age = 200; // Throws an exception
```
**[⬆ Back to Top](#table-of-contents)**
20. ### Promises
A promise is an object which represent the eventual completion or failure of an asynchronous operation.
It is in one of these states:
**pending:** Represents initial state, neither fulfilled nor rejected.
**fulfilled:** Indicates that the operation is completed successfully.
**rejected:** Indicates that the operation is failed.
A promise is said to be settled if it is either fulfilled or rejected, but not pending. The instance methods `promise.then()`, `promise.catch()`, and `promise.finally()` are used to associate further action with a promise that becomes settled. And these methods also return a newly generated promise object, which can optionally be used for chaining.
The promise chaining structure would be as below,
```js
const promise = new Promise(function(resolve, reject) {
setTimeout(() => resolve(1), 1000);
});
promise.then(function(result) {
console.log(result); // 1
return result * 2;
}).then(function(result) {
console.log(result); // 2
return result * 3;
}).then(function(result) {
console.log(result); // 6
return result * 4;
}).catch(function(error){
console.log(error);
});
```
**[⬆ Back to Top](#table-of-contents)**
21. ### Reflect
Reflection is the ability of a code to inspect and manipulate variables, properties, and methods of objects at runtime. JavaScript already provides `Object.keys(), Object.getOwnPropertyDescriptor(), and Array.isArray()` methods as classic refection features. In ES6, it has been officially provided through Reflect object. Reflect is a new global object which is used to call methods, construct objects, get and set properties, manipulate and extend properties.
Unlike most global objects, Reflect is not a constructor. i.e, You cannot use Reflect with the new operator or invoke the Reflect as a function. It is similar to Math and JSON objects in which all the methods of this object are static.
Let's see the usage of Reflect API with below examples,
1. **Creating objects using Reflect.construct();**
The `construct()` method behaves like the regular new operator, but as a function. It is equivalent to calling new target(...args) with an option to specify a different prototype. The syntax looks like as below,
```js
Reflect.construct(target, args [, newTarget]);
```
The method has below parameters,
1. target: The target function to call.
2. argumentsList: An array-like object specifying the arguments with which target should be called.
3. newTarget: The constructor whose prototype should be used. This is an optional parameter. i.e, If newTarget is not present, its value defaults to target.
**Example:**
```js
class User {
constructor(firstName, lastName) {
this.firstName = firstName;
this.lastName = lastName;
}
get fullName() {
return `${this.firstName} ${this.lastName}`;
}
};
let args = ['John', 'Emma'];
let john = Reflect.construct(
User,
args
);
console.log(john instanceof User);
console.log(john.fullName); // John Doe
```
2. **Calling a function using Reflect.apply():**
Prior to ES6, you can invoke a function with a specified `this` value and arguments by using the `Function.prototype.apply()` method.
For example, you can call `max()` static method of Math object,
```js
const max = Function.prototype.apply.call(Math.max, Math, [100, 200, 300]);
console.log(max);
```
In ES6, Reflect.apply() provides the same features as Function.prototype.apply() but in a less verbose syntax.
```js
const max = Reflect.apply(Math.max, Math, [100, 200, 300]);
console.log(max);
```
3. **Defining a property using Reflect.defineProperty():**
The `Reflect.defineProperty()` method is similar to `Object.defineProperty()` but it returns a Boolean value indicating whether or not the property was defined successfully instead of throwing an exception.
The syntax of this method looks like below,
```js
Reflect.defineProperty(target, propertyName, propertyDescriptor)
```
Let's define the age property on user object,
```js
class User {
constructor(firstName, lastName) {
this.firstName = firstName;
this.lastName = lastName;
}
get fullName() {
return `${this.firstName} ${this.lastName}`;
}
};
let john = new User('John', 'Resig');
if (Reflect.defineProperty(john, 'age', {
writable: true,
configurable: true,
enumerable: false,
value: 33,
})) {
console.log(john.age);
} else {
console.log('Cannot define the age property on the user object.');
}
```
4. **Delete property using Reflect.deleteProperty():**
The `Reflect.deleteProperty()` method is used to delete properties like the delete operator but as a function. It returns Boolean value indicating whether or not the property was successfully deleted.
```js
const user = {
name: 'John',
age: 33
};
console.log(Reflect.deleteProperty(user, 'age')); // true
console.log(user.age); // undefined
```
5. **Get property of an object using Reflect.get():**
The `Reflect.get` method is used to get a property on an object like the property accessor syntax but as a function.
```js
const user = {
name: 'John',
age: 33
};
console.log(Reflect.get(user, 'age')); // 33
```
6. **:**
**[⬆ Back to Top](#table-of-contents)**
22. ### Binary and Octal
ES5 provided numeric literals in octal (prefix 0), decimal (no prefix), and hexadecimal ( 0x) representation. ES6 added support for binary literals and improvements on octal literals.
**1. Binary literals:**
Prior to ES5, JavaScript didn’t provide any literal form of binary numbers. So you need to use a binary string with the help of `parseInt()`
```js
const num = parseInt('110',2);
console.log(num); // 6
```
Whereas ES6 added support for binary literals using the **0b** prefix followed by a sequence of binary numbers (i.e, 0 and 1).
```js
const num = 0b110;
console.log(num); // 6
```
**2. Octal literals:**
In ES5, to represent an octal literal, you use the zero prefix (0) followed by a sequence of octal digits (from 0 to 7).
```js
const num = 055;
console.log(num); // 45
let invalidNum = 058;
console.log(invalidNum); // treated as decimal 58
```
Whereas ES6 represents the octal literal by using the prefix **0o** followed by a sequence of octal digits from 0 through 7.
```js
const num = 055;
console.log(num); // 45
const invalidNum = 028;
console.log(invalidNum); // treated as decimal 28
```
Remember If you use an invalid number in the octal literal, JavaScript will throw a SyntaxError as below,
```js
const invalidNum = 028;
console.log(invalidNum); // SyntaxError
```
**[⬆ Back to Top](#table-of-contents)**
23. ### Proper Tail Calls
**Proper tail call(PTC)** is a technique where the program or code will not create additional stack frames for a recursion when the function call is a tail call.
For example, the below classic or head recursion of factorial function relies on stack for each step. Each step need to be processed upto `n * factorial(n - 1)`
```js
function factorial(n) {
if (n === 0) {
return 1
}
return n * factorial(n - 1)
}
console.log(factorial(5)); //120
```
But if you use Tail recursion functions, they keep passing all the necessary data it needs down the recursion without relying on the stack.
```js
function factorial(n, acc = 1) {
if (n === 0) {
return acc
}
return factorial(n - 1, n * acc)
}
console.log(factorial(5)); //120
```
The above pattern returns the same output as first one. But the accumulator keeps track of total as an argument without using stack memory on recursive calls.
The browsers which supports PTC do not generate stack overflow instead shows Infinity with below inputs,
```js
console.log(factorial(10));
console.log(factorial(100));
console.log(factorial(1000));
console.log(factorial(10000));
```
**[⬆ Back to Top](#table-of-contents)**
24. ### Array find methods
ES6 introduced few array methods and two of them are `Array.find()` and `Array.findIndex()`.
**Array.find()**
This method returns the value of the first element in an array that satisfies the given test. Let's take an example of array with all even elements except one element and use `find` method to find the odd element.
```js
let arr = [2, 4, 5, 6, 8, 10];
function isOdd(i) {
return i % 2 !== 0;
}
console.log(arr.find(isOdd)); // 5
```
**Array.findIndex()**
This method returns the index of the first element in the array that satisfies the given test. Let's take an example of array with all even elements except one element and use `findIndex` method to find the index of odd element.
```js
let arr = [2, 4, 5, 6, 8, 10];
function isOdd(i) {
return i % 2 !== 0;
}
console.log(arr.findIndex(isOdd)); //2
```
**[⬆ Back to Top](#table-of-contents)**
## ES2016 Or ES7
ES2015/ES6 introduced a huge set of new features. But ECMAScript 2016 Or ES7 introduced only two new features:
1. Array.prototype.includes()
2. Exponentiation operator
1. ### Array Includes
Prior to ES7, you have to use `indexOf` method and compare the result with '-1' to check whether an array element contains particular element or not.
```js
const array = [1,2,3,4,5,6];
if(array.indexOf(5) > -1 ){
console.log("Found an element");
}
```
Whereas in ES7, `array.prototype.includes()` method is introduced as a direct approach to determine whether an array includes a certain value among its entries or not.
```js
const array = [1,2,3,4,5,6];
if(array.includes(5)){
console.log("Found an element");
}
```
In addition to this, `Array.prototype.includes()` handles NaN and Undefined values better than `Array.prototype.indexOf()` methods. i.e, If the array contains NaN and Undefined values then `indexOf()` does not return correct index while searching for NaN and Undefined.
```js
let numbers = [1, 2, 3, 4, NaN, ,];
console.log(numbers.indexOf(NaN)); // -1
console.log(numbers.indexOf(undefined)); // -1
```
On the otherhand, `includes` method is able to find these elements
```js
let numbers = [1, 2, 3, 4, NaN, ,];
console.log(numbers.includes(NaN)); // true
console.log(numbers.includes(undefined)); // true
```
**[⬆ Back to Top](#table-of-contents)**
2. ### Exponentiation Operator
The older versions of javascript uses `Math.pow` function to find the exponentiation of given numbers. ECMAScript 2016 introduced the exponentiation operator, **(similar to other languages such as Python or F#) to calculate the power computation in a clear representation using infix notation.
```js
//Prior ES7
const cube = x => Math.pow(x, 3);
console.log(cube(3)); // 27
//Using ES7
const cube1 = x => x ** 3;
console.log(cube1(3)); // 27
```
**[⬆ Back to Top](#table-of-contents)**
## ES2017 Or ES8
1. ### Async functions
In ES6, Promises were introduced to solve the famous callback hell problem. When a series of nested asynchronous functions need to be executed in order, it leads to a callback hell
```js
function task() {
task1((response1) => {
task2(response1, (response2) => {
task3(response2, (response3) => {
// etc...
};
});
});
}
```
But the Chained Promises creates complex flow for asynchronous code.
Async functions were introduced as a combination of promises and generators to give us the possibility of writing asynchronous in a synchronous manner. i.e, This function is going to be declared with the `async` keyword which enable asynchronous, promise-based behavior to be written in a cleaner style by avoiding promise chains. These functions can contain zero or more `await` expressions.
Let's take a below async function example,
```js
async function logger() {
let data = await fetch('http://someapi.com/users'); // pause until fetch returns
console.log(data)
}
logger();
```
**[⬆ Back to Top](#table-of-contents)**
2. ### Object values
Similar to Object.keys which iterate over JavaScript object’s keys, Object.values will do the same thing on values. i.e, The Object.values() method is introduced to returns an array of a given object's own enumerable property values in the same order as `for...in` loop.
```js
const countries = {
IN: 'India',
SG: 'Singapore',
}
Object.values(countries) // ['India', 'Singapore']
```
By the way, non-object argument will be coerced to an object
```js
console.log(Object.values(['India', 'Singapore'])); // ['India', 'Singapore']
console.log(Object.values('India')); // ['I', 'n', 'd', 'i', 'a']
```
**[⬆ Back to Top](#table-of-contents)**
3. ### Object entries
The `Object.entries()` method is introduced to returns an array of a given object's own enumerable string-keyed property [key, value] pairsin the same order as `for...in` loop.
```js
const countries = {
IN: 'India',
SG: 'Singapore',
}
Object.entries(countries) // [["IN", "India"], ["SG", "Singapore"]]
```
By the way, non-object argument will be coerced to an object
```js
const countriesArr = ['India', 'Singapore'];
console.log(Object.entries(countriesArr)); // [ ['0', 'India'], ['1', 'Singapore']]
const country = 'India';
console.log(Object.entries(country)); // [["0", "I"], ["1", "n"], ["2", "d"], ["3", "i"], ["4", "a"]]
console.log(Object.entries(100)); // [], an empty array for any primitive type because it won't have any own properties
```
**[⬆ Back to Top](#table-of-contents)**
4. ### Object property descriptors
Property descriptors describe the attributes of a property. The `Object.getOwnPropertyDescriptors()` method returns all own property descriptors of a given object.
It provides the below attributes,
1. **value:** The value associated with the property (data descriptors only).
2. **writable:** true if and only if the value associated with the property may be changed
3. **get:** A function which serves as a getter for the property.
4. **set:** A function which serves as a setter for the property.
5. **configurable:** true if and only if the type of this property descriptor may be changed or deleted.
6. **enumerable:** true if and only if this property shows up during enumeration of the property.
The usage of finding property descriptors for any property seems to be as below,
```js
const profile = {
age: 42
};
const descriptors = Object.getOwnPropertyDescriptors(profile);
console.log(descriptors); // {age: {configurable: true, enumerable: true, writable: true }}
```
**[⬆ Back to Top](#table-of-contents)**
5. ### String padding
Some strings and numbers(money, date, timers etc) need to be represented in a particular format. Both `padStart() & padEnd()` methods introduced to pad a string with another string until the resulting string reaches the supplied length.
1. **padStart():** Using this method, padding is applied to the left or beginning side of the string.
For example, you may want to show only the last four digits of credit card number for security reasons,
```js
const cardNumber = '01234567891234';
const lastFourDigits = cardNumber.slice(-4);
const maskedCardNumber = lastFourDigits.padStart(cardNumber.length, '*');
console.log(maskedCardNumber); // expected output: "**********1234"
```
2. **padEnd():** Using this method, padding is applied to the right or ending side of the string.
For example, the profile information padded for label and values as below
```js
const label1 = "Name";
const label2 = "Phone Number";
const value1 = "John"
const value2 = "(222)-333-3456";
console.log((label1 + ': ').padEnd(20, ' ') + value1); // Name: John
console.log(label2 + ": " + value2); // Phone Number: (222)-333-3456
```
**[⬆ Back to Top](#table-of-contents)**
6. ### Shared memory and atomics
The Atomics is a global object which provides atomic operations to be performed as static methods. They are used with SharedArrayBuffer(fixed-length binary data buffer) objects. The main use cases of these methods are,
1. **atomic operations:** When memory is shared, multiple threads can read and write the same data in memory. So there would be a chance of loss of data. But atomic operations make sure that predictable values are written and read, that operations are finished before the next operation starts and that operations are not interrupted.
It provides static methods such as add, or, and, xor, load, store, isLockFree etc as demonstrated below.
```js
const sharedMemory = new SharedArrayBuffer(1024);
const sharedArray = new Uint8Array(sharedMemory);
sharedArray[0] = 10;
Atomics.add(sharedArray, 0, 20);
console.log(Atomics.load(sharedArray, 0)); // 30
Atomics.sub(sharedArray, 0, 10);
console.log(Atomics.load(sharedArray, 0)); // 20
Atomics.and(sharedArray, 0, 5);
console.log(Atomics.load(sharedArray, 0)); // 4
Atomics.or(sharedArray, 0, 1);
console.log(Atomics.load(sharedArray, 0)); // 5
Atomics.xor(sharedArray, 0, 1);
console.log(Atomics.load(sharedArray, 0)); // 4
Atomics.store(sharedArray, 0, 10); // 10
Atomics.compareExchange(sharedArray, 0, 5, 10);
console.log(Atomics.load(sharedArray, 0)); // 10
Atomics.exchange(sharedArray, 0, 10);
console.log(Atomics.load(sharedArray, 0)); //10
Atomics.isLockFree(1); // true
```
2. **waiting to be notified:**
Both `wait()` and `notify()` methods provides ways for waiting until a certain condition becomes true and are typically used as blocking constructs.
Let's demonstrate this functionality with reading and writing threads.
First define a shared memory and array
```js
const sharedMemory = new SharedArrayBuffer(1024);
const sharedArray = new Int32Array(sharedMemory);
```
A reading thread is sleeping and waiting on location 0 which is expected to be 10. You can observe a different value after the value overwritten by a writing thread.
```js
Atomics.wait(sharedArray, 0, 10);
console.log(sharedArray[0]); // 100
```
Now a writing thread stores a new value(e.g, 100) and notifies the waiting thread,
```js
Atomics.store(sharedArray, 0, 100);
Atomics.notify(sharedArray, 0, 1);
```
7. ### Trailing commas
Trailing commas are allowed in parameter definitions and function calls
```js
function func(a,b,) { // declaration
console.log(a, b);
}
func(1,2,); // invocation
```
But if the function parameter definition or function call only contains a comma, a syntax error will be thrown
```js
function func1(,) { // SyntaxError: missing formal parameter
console.log('no args');
};
func1(,); // SyntaxError: expected expression, got ','
```
**Note:** Trailing commas are not allowed in Rest Parameters and JSON.
**[⬆ Back to Top](#table-of-contents)**
## ES2018 Or ES9
1. ### Async iterators
ECMAScript 6 provides built-in support for synchronously iterating over data using iterators. Both strings and collections objects such as Set, Map, and Array come with a Symbol.iterator property which makes them iterable.
```js
const arr = ['a', 'b', 'c', 'd'];
const syncIterator = arr[Symbol.iterator]();
console.log(syncIterator.next()); // {value: a, done: false}
console.log(syncIterator.next()); // {value: b, done: false}
console.log(syncIterator.next()); // {value: c, done: false}
console.log(syncIterator.next()); // {value: d, done: false}
console.log(syncIterator.next()); // {value: undefined, done: true}
```
But these iterators are only suitable for representing synchronous data sources.
In order to access asynchronous data sources, ES2018 introduced the AsyncIterator interface, an asynchronous iteration statement (for-await-of), and async generator functions.
**[⬆ Back to Top](#table-of-contents)**
2. ### Object rest and spread operators
ES2015 or ES6 introduced both rest parameters and spread operators to convert arguments to array and vice versa using three-dot(...) notation.
1. Rest parameters can be used to convert function arguments to an array
```js
function myfunc(p1, p2, ...p3) {
console.log(p1, p2, p3); // 1, 2, [3, 4, 5, 6]
}
myfunc(1, 2, 3, 4, 5, 6);
```
2. The spread operator works in the opposite way by converting an array into separate arguments in order to pass to a function
```js
const myArray = [10, 5, 25, -100, 200, -200];
console.log( Math.max(...myArray) ); // 200
```
ES2018 enables this rest/spread behavior for objects as well.
1. You can pass object to a function
```js
function myfunc1({ a, ...x }) {
console.log(a, x); // 1, { b: 2, c: 3, d:4 }
}
myfunc1({
a: 1,
b: 2,
c: 3,
d: 4
});
```
2. The spread operator can be used within other objects
```js
const myObject = { a: 1, b: 2, c: 3, d:4 };
const myNewObject = { ...myObject, e: 5 }; // { a: 1, b: 2, c: 3, d: 4, e: 5 }
```
**[⬆ Back to Top](#table-of-contents)**
3. ### Promise finally
Sometimes you may need to avoid duplicate code in the then() and catch() methods.
```js
myPromise
.then(result => {
// process the result and then clean up the resources
})
.catch(error => {
// handle the error and then clean up the resources
});
```
The `finally()` method is useful if you want to do some processing or resource cleanup once the promise is settled(i.e either fulfilled or rejected).
Let's take a below example to hide the loading spinner after the data is fetched and processed.
```js
let isLoading = true;
fetch('http://somesite.com/users')
.then(data => data.json())
.catch(err => console.error(err))
.finally(() => {
isLoading = false;
console.log('Finished loading!!');
})
```
**[⬆ Back to Top](#table-of-contents)**
## ES2019 Or ES10
1. ### Array flat and flatMap
Prior to ES2019, you need to use `reduce() or concat()` methods to get a flat array.
```js
function flatten(arr) {
const flat = [].concat(...arr);
return flat.some(Array.isArray) ? flatten(flat) : flat;
}
flatten([ [1, 2, 3], ['one', 'two', 'three', [22, 33] ], ['a', 'b', 'c'] ]);
```
In ES2019, the `flat()` method is introduced to 'flattens' the nested arrays into the top-level array. The functionality of this method is similar to Lodash's `_.flattenDepth()` function. This method accepts an optional argument that specifies the number of levels a nested array should be flattened and the default nested level is 1.
**Note:** If there are any empty slots in the array, they will be discarded.
```js
const numberArray = [[1, 2], [[3], 4], [5, 6]];
const charArray = ['a', , 'b', , , ['c', 'd'], 'e'];
const flattenedArrOneLevel = numberArray.flat(1);
const flattenedArrTwoLevel = numberArray.flat(2);
const flattenedCharArrOneLevel = charArray.flat(1);
console.log(flattenedArrOneLevel); // [1, 2, [3], 4, 5, 6]
console.log(flattenedArrTwoLevel); // [1, 2, 3, 4, 5, 6]
console.log(flattenedCharArrOneLevel); // ['a', 'b', 'c', 'd', 'e']
```
Whereas, **flatMap()** method combines `map()` and `flat()` into one method. It first creates a new array with the return value of a given function and then concatenates all sub-array elements of the array.
```js
const numberArray1 = [[1], [2], [3], [4], [5]];
console.log(numberArray1.flatMap(value => [value * 10])); // [10, 20, 30, 40, 50]
```
**[⬆ Back to Top](#table-of-contents)**
2. ### Object fromEntries
In JavaScript, it is very commonn to transforming data from one format. ES2017 introduced `Object.entries()` method to objects into arrays.
**Object to Array:**
```js
const obj = {'a': '1', 'b': '2', 'c': '3' };
const arr = Object.entries(obj);
console.log(arr); // [ ['a', '1'], ['b', '2'], ['c', '3'] ]
```
But if you want to get the object back from an array then you need iterate and convert it as below,
```js
const arr = [ ['a', '1'], ['b', '2'], ['c', '3'] ];
let obj = {}
for (let [key, val] of arr) {
obj[key] = val;
}
console.log(obj);
```
We need a straightforward way to avoid this iteration. In ES2019, `Object.fromEntries()` method is introduced which performs the reverse of `Object.entries()` behavior. The above loop can be avoided easily as below,
**Iterable( e.g Array or Map) to Object**
```js
const arr = [ ['a', '1'], ['b', '2'], ['c', '3'] ];
const obj = Object.fromEntries(arr);
console.log(obj); // { a: "1", b: "2", c: "3" }
```
One of the common case of this method usage is working with query params of an URL,
```js
const paramsString = 'param1=foo¶m2=baz';
const searchParams = new URLSearchParams(paramsString);
Object.fromEntries(searchParams); // => {param1: "foo", param2: "baz"}
```
**[⬆ Back to Top](#table-of-contents)**
3. ### String trimStart and trimEnd
In order to make consistency with padStart/padEnd, ES2019 provided the standard functions named as `trimStart` and `trimEnd` to trim white spaces on the beginning and ending of a string. However for web compatilibity(avoid any breakage) `trimLeft` and `trimRight` will be an alias for `trimStart` and `trimEnd` respectively.
Let's see the usage with an example,
```js
//Prior ES2019
let messageOne = " Hello World!! ";
console.log(messageOne.trimLeft()); //Hello World!!
console.log(messageOne.trimRight()); // Hello World!!
//With ES2019
let messageTwo = " Hello World!! ";
console.log(messageTwo.trimStart()); //Hello World!!
console.log(messageTwo.trimEnd()); // Hello World!!
```
**[⬆ Back to Top](#table-of-contents)**
4. ### Symbol description
While creating symbols, you also can add a description to it for debugging purposes. But there was no method to access the description directly before ES2019. Considering this, ES2019 introduced a read-only description property to retrieve a string containing the description of the Symbol.
This gives the possibility to access symbol description for different variations of Symbol objects
```js
console.log(Symbol('one').description); // one
console.log(Symbol.for('one').description); // "one"
console.log(Symbol('').description); // ''
console.log(Symbol().description); // unefined
console.log(Symbol.iterator.description); // "Symbol.iterator"
```
**[⬆ Back to Top](#table-of-contents)**
5. ### Optional catch binding
Prior to ES9, if you don't need `error` variable and omit the same variable then catch() clause won't be invoked. Also, the linters complain about unused variables. Inorder to avoid this problem, the optional catch binding feature is introduced to make the binding parameter optional in the catch clause. If you want to completely ignore the error or you already know the error but you just want to react to that the this feature is going to be useful.
Let's see the below syntax difference between the versions,
```js
// With binding parameter( a.age - b.age);
```
**[⬆ Back to Top](#table-of-contents)**
8. ### Function.toString()
Functions have an instance method called `toString()` which return a string to represent the function code. Previous versions of ECMAScript removes white spaces,new lines and comments from the function code but it has been retained with original source code in ES2020.
```js
function sayHello(message) {
let msg = message;
//Print message
console.log(`Hello, ${msg}`);
}
console.log(sayHello.toString());
// function sayHello(message) {
// let msg = message;
// //Print message
// console.log(`Hello, ${msg}`);
// }
```
**[⬆ Back to Top](#table-of-contents)**
9. ### Private Class Variables
In ES6, the classes are introduced to create reusable modules and variables are declared in clousure to make them private. Where as in ES2020, private class variables are introduced to allow the variables used in the class only. By just adding a simple hash symbol in front of our variable or function, you can reserve them entirely for internal to the class.
```js
class User {
#message = "Welcome to ES2020"
login() { console.log(this.#message) }
}
const user = new User()
user.login() // Welcome to ES2020
console.log(user.#message) // Uncaught SyntaxError: Private field '#
```
**Note:** As shown in the above code, If you still try to access the variable directly from the object then you will receive syntax error.
**[⬆ Back to Top](#table-of-contents)**
## ES2020 Or ES11
ES2020 is the current newer version of ECMAScript corresponding to the year 2020. This is the eleventh edition of the ECMAScript Language Specification. Even though this release doesn't bring as many features as ES6, it included some really useful features.
Most of these features already supported by some browsers and try out with babel parser support for unsupported features. This edition is set for final approval by the ECMA general assembly in June, 2020. The [ECMAScript 2020 (ES2020) language specification](https://tc39.es/ecma262/2020/) is ready now.
1. ### BigInt
In earlier JavaScript version, there is a limitation of using the Number type. i.e, You cannot safely represent integer values(`Number` primitive) larger than pow(2, 53). In ES2020,
`BigInt` is introduced as the 7th primitive type to represent whole numbers(integers with arbitrary precision) larger than pow(2, 53) - 1(or 9007199254740991 or Number.MAX_SAFE_INTEGER). This is been created by appending `n` to the end of an integer literal or by calling the function BigInt().
```js
// 1. Current number system
const max = Number.MAX_SAFE_INTEGER;
console.log(max + 1) // 9007199254740992
console.log(max + 2) // 9007199254740992
// 2. BigInt representation
const bigInt = 9007199254740991n;
const bigIntConstructorRep = BigInt(9007199254740991); // 9007199254740991n
const bigIntStringRep = BigInt("9007199254740991"); // 9007199254740991n
// 3. Typeof usage
console.log(typeof 1)// number
console.log(typeof 1n)// bigint
console.log(typeof BigInt('1'))// bigint
// 4. Operators
const previousMaxNum = BigInt(Number.MAX_SAFE_INTEGER);
console.log(previousMaxNum + 2n); //9007199254740993n (this was not possible before)
console.log(previousMaxNum -2n); //9007199254740990n
console.log(previousMaxNum * 2n); //18014398509481982n
console.log(previousMaxNum % 2n); //1n
console.log(previousMaxNum / 2n); // 4503599627370495n
// 5. comparison
console.log(1n === 1); // false
console.log(1n === BigInt(1)); // true
console.log(1n == 1); // true
```
**[⬆ Back to Top](#table-of-contents)**
2. ### Dynamic Import
Static imports supports some of the important use cases such as static analysis, bundling tools, and tree shaking, it is also it's desirable to be able to dynamically load parts of a JavaScript application at runtime.
The new feature `dynamic import` is introduced to load a module conditionally or on demand. Since it returns a promise for the module namespace object of the requested module, the module can be resolved or import can now be assigned to a variable using async/await as below
```js
const moduleSpecifier = './message.js';
import(moduleSpecifier)
.then((module) => {
module.default(); // Hello, default export
module.sayGoodBye(); //Bye, named export
})
.catch(err => console.log('loading error'));
```
```js
(async function() {
const moduleSpecifier = './message.js';
const messageModule = await import(moduleSpecifier);
messageModule.default(); // Hello, default export
messageModule.sayGoodBye(); //Bye, named export
})();
```
and the imported module appears with both default and named exports
```js
export default () => {
return "Hello, default export";
}
export const sayGoodBye = () => {
return "Bye, named export"
}
```
**Note:** Dynamic import does not require scripts of `type="module"`
**[⬆ Back to Top](#table-of-contents)**
3. ### Nullish Coalescing Operator
The nullish coalescing operator (??) is a logical operator that returns its right-hand side operand when its left-hand side operand is `null` or `undefined`, and otherwise returns its left-hand side operand. This operator replaces `||` operator to provide default values if you treat empty value or '', 0 and NaN as valid values. This is because the logical OR(||) operator treats(empty value or '', 0 and NaN) as falsy values and returns the right operand value which is wrong in this case. Hence, this operator truely checks for `nullish` values instead `falsy` values.
```js
let vehicle = {
car: {
name: "",
speed: 0
}
};
console.log(vehicle.car.name || "Unknown"); // Unknown
console.log(vehicle.car.speed || 90); // 90
console.log(vehicle.car.name ?? "Unknown"); // ""(empty is valid case for name)
console.log(vehicle.car.speed ?? 90); // 0(zero is valid case for speed)
```
In a short note, nullish operator returns a non-nullish value and || operator returns truthy values.
**[⬆ Back to Top](#table-of-contents)**
4. ### String matchAll
There is `String#match` method to get all the matches of a string against a regular expression by iterating for each match. However this method gives you the substrings that match.
The `String#matchAll()` is a new method added to String prototype, which returns an iterator of all results matching a string against a regular expression.
```js
const regex = /t(e)(st(\d?))/g;
const string = 'test1test2';
const matchesIterator = string.matchAll(regex);
Array.from(matchesIterator, result => console.log(result));
```
When you this code in browser console, the matches iterator produces an array for each match including the capturing groups with a few extras.
```cmd
["test1", "e", "st1", "1", index: 0, input: "test1test2", groups: undefined]
["test2", "e", "st2", "2", index: 5, input: "test1test2", groups: undefined]
```
**[⬆ Back to Top](#table-of-contents)**
5. ### Optional chaining
In JavaScript, Long chains of property accesses is quite error-prone if any of them evaluates to `null` or `undefined` value. Also, it is not a good idea to check property existence on each item which in turn leads to a deeply-nested structured `if` statements.
Optional chaining is a new feature that can make your JavaScript code look cleaner and robust by appending(?.) operator to stop the evaluation and return undefined if the item is undefined or null.
By the way, this operator can be used together with nullish coalescing operator to provide default values
```js
let vehicle = {
};
let vehicle1 = {
car: {
name: 'ABC',
speed: 90
}
};
console.log(vehicle.car.name); // TypeError: Cannot read property 'name' of undefined
console.log(vehicle.car?.name); // Undefined
console.log(vehicle.car?.speed); // Undefined
console.log(vehicle1.car?.name); // ABC
console.log(vehicle1.car?.speed); // 90
console.log(vehicle.car?.name ?? "Unknown"); // Unknown
console.log(vehicle.car?.speed ?? 90); // 90
```
**[⬆ Back to Top](#table-of-contents)**
6. ### Promise.allSettled
It is really helpful to log(especially to debug errors) about each promise when you are handling multiple promises. The `Promise.allSettled()` method returns a new promise that resolves after all of the given promises have either fulfilled or rejected, with an array of objects describing the outcome of each promise.
```js
const promise1 = new Promise((resolve, reject) => setTimeout(() => resolve(100), 1000));
const promise2 = new Promise((resolve, reject) => setTimeout(reject, 1000));
Promise.allSettled([promise1, promise2]).then(data => console.log(data)); // [
// Object { status: "fulfilled", value: 100},
// Object { status: "rejected", reason: undefined}
// ]
```
As per the output, each outcome object returns `status` field which denotes either "fulfilled"(value present) or "rejected"(reason present)
**[⬆ Back to Top](#table-of-contents)**
7. ### globalThis
Prior to ES2020, you need to write different syntax in different JavaScript environments(cross-platforms) just to access the global object. It is really a hard time for developers because you need to use `window, self, or frames` on the browser side, `global` on the nodejs, `self` on the web workers side.
On the other hand, `this` keyword can be used inside functions for non-strict mode but it gives undefined in strict mode. If you think about `Function('return this')()` as a solution for above environments, it will fail for CSP enabled environments(where eval() is disabled).
In the older versions, you can use es6-shim as below,
```js
var getGlobal = function () {
if (typeof self !== 'undefined') { return self; }
if (typeof window !== 'undefined') { return window; }
if (typeof global !== 'undefined') { return global; }
throw new Error('unable to locate global object');
};
var globals = getGlobal();
if (typeof globals.setTimeout !== 'function') {
console.log('no setTimeout in this environment or runtime');
}
```
In ES2020, `globalThis` property is introduced to provide a standard way of accessing the global this value across environments.
```js
if (typeof globalThis.setTimeout !== 'function') {
console.log('no setTimeout in this environment or runtime');
}
```
8. ### import.meta
The `import.meta` object was created by the ECMAScript implementation with a null prototype to get context-specific metadata about a JavaScript module.
Let's say you are trying to load `my-module` from a script,
```js
```
Now you can access meta information(base URL of the module) about the module using the import.meta object
```js
console.log(import.meta); // { url: "file:///home/user/my-module.js" }
```
The above URL can be either URL from which the script was obtained (for external scripts), or the document base URL of the containing document (for inline scripts).
**Note:** Remember `import` is not really an object but `import.meta` is provided as an object which is extensible, and its properties are writable, configurable, and enumerable.
9. ### for..in order
Prior to ES2020, the specifications did not specify in which order for (a in b) should run. Even though most of the javascript engines/browsers loop over the properties of an object in the order in which they were defined, it is not the case with all scenarios. This has been officially standardized in ES2020.
```js
var object = {
'a': 2,
'b': 3,
'c': 4
}
for(let key in object) {
console.log(key); // a b c
}
```
**[⬆ Back to Top](#table-of-contents)**
## ES2021 Or ES12
ECMAScript 2021 or ES12 has been released in mid of 2021 with few important features which can be used JavaScript.
1. ### replaceAll
The new `replaceAll()` method from `String` prototype is used to replace all the occurrences of a string from another string value. Earlier it was not possible to replace all the instances of a substring without the use of regex.
**Before ES2021**
```javascript
console.log('10101010'.replace(new RegExp('0', 'g'), '1')); // 11111111
console.log('01010101'.replace(/0/g, '1')); // 11111111
```
**After ES2021**
```javascript
console.log('10101010'.replaceAll('0', '1')); // 11111111
console.log('01010101'.replaceAll('0', '1')); // 11111111
```
2. ### promise.any
The new `promise.any` method takes multiple promises and resolves to the value of the first promise which is successfully fulfilled.
```javascript
let promise1 = new Promise((resolve) => setTimeout(resolve, 100, 'Resolves after 100ms'));
let promise2 = new Promise((resolve) => setTimeout(resolve, 200, 'Resolves after 200ms'));
let promise3 = new Promise((resolve, reject) => setTimeout(reject, 0) );
let promises = [promise1, promise2, promise3];
Promise.any(promises)
.then( value => console.log(value)); // Resolves after 100ms
```
In case none of the promises resolved then it throws `AggregateError` exception.
```javascript
(async () => {
try {
const output = await Promise.any([
Promise.reject('Error 1'),
Promise.reject('Error 2'),
Promise.reject('Error 3'),
]);
console.log(`Output: ${output}`);
} catch (err) {
console.log(`Error: ${err.errors}`);
}
})();
// Error: Error1,Error2,Error3
```
3. ### WeakRef
WeakRef provides two new pieces of functionality
1. creating weak references to objects with the WeakRef class
2. running user-defined finalizers after objects are garbage-collected, with the FinalizationRegistry class
**WeakRef:**
weak reference is a reference to an object that doesn’t prevent garbage collection if it is the only reference to the object in the memory.It’s useful when we don’t want to keep the object in memory forever(e.g, WebSocket). The main use of weak references is to implement caches or mappings to large objects for which you don't need to keep it in memory for rarely used objects.
Prior to ES12, WeakMaps and WeakSets are the only way to kind-of-weakly reference an object in JavaScript. Whereas WeakRef in ES12 provides actual weak references, enabling a window into the lifetime of an object.
Let's see an example of a weak reference object using `WeakRef` constructor and read the reference using `deref()` method
```javascript
const myObject = new WeakRef({
name: ‘Sudheer’,
age: 34
});
console.log(myObject.deref()); //output: {name: “Sudheer”, age: 35}
console.log(myObject.deref().name); //output: Sudheer
```
**Finalizers:**
A `FinalizationRegistry` object lets you request a callback when an object is garbage-collected. It works as a cleanup callback.
```javascript
// Create new FinalizationRegistry:
const reg = new FinalizationRegistry((val) => {
console.log(val);
});
(() => {
// Create new object:
const obj = {}
// Register finalizer for the "obj" as first argument and value for callback function as second argument:
reg.register(obj, 'obj has been garbage-collected.')
})();
```
**Note:** The finalization callback does not run immediately after garbage-collecting the event listener, so don't use it for important logic or metrics.
4. ### Numeric Separators
Numeric separators are helpful to read large numbers(or numeric literals) in JavaScript by providing separation between digits using underscores(_). In otherwords, numeric literals are more readable by creating a visual separation between groups of digits.
For example, one billion and one trillion becomes more readable with _ numeric separator,
```javascript
const billion = 1000_000_000;
console.log(billion); // 1000000000
const trillion = 1000_000_000_000n; // BigInt number
console.log(trillion); // 1000000000000
```
It can be used for binary and hex literals as well.
```javascript
const binaryLiteral = 0b1010_1010;
console.log(binaryLiteral);
const hexLiteral = 0xFF_FF_FF_FF;
console.log(hexLiteral);
```
**Note:** The separator can be placed anywhere within the number for readability purpose.
5. ### Logical Operators
Logical assignment operators combines the logical operations(&&, || or ??) with assignment. They are quite useful for assigning default values to variables.
**&&=:**
The `&&=` operator performs the assignment only when the left operand is truthy.
```javascript
let x = 10;
let y = 20;
x &&= y;
console.log(x); // 20
```
The above logical assignment operation can be expanded to:
```javascript
x = x && (x = y);
(OR)
if (x) {
x = y;
}
```
**||=:**
The ||= operator performs the assignment only when the left operand is falsy.
```javascript
let x = 0;
let y = 20;
x ||= y;
console.log(x); // 20
```
The above logical assignment operation can be expanded to:
```javascript
x = x || (x = y);
(OR)
if (!x) {
x = y;
}
```
**??=:**
The ??= operator performs the assignment only when the left operand is null or undefined.
```javascript
let x;
let y = 1;
x ??= y;
console.log(x); // 1
```
The above logical assignment operation can be expanded to:
```javascript
x ?? (x = y);
(OR)
if (!x) {
x = y;
}
```
**[⬆ Back to Top](#table-of-contents)**
## ES2022 Or ES13
ECMAScript 2022 or ES13 has been released in the month of June 2022 with some of important features in JavaScript.
1. ### Top-level await
In ES2022, it is possible to use `await` outside of the asynchronous (async) function scope, which makes it easier to use at the module level. This feaure delays the execution of current and parent modules until the imported module is loaded.
Let's take an example of `await` usage prior to ES2022,
```javascript
import posts from './posts';
const getPosts = async() => {
let posts = await posts();
return posts;
}
```
The usage of `await` is straightword with ES2022 as below,
```javascript
let posts = await posts();
```
There are few use cases to know the benefits of this top-level await.
#### Usecases
1. **Dynamic dependency pathing:**
When you have a dynamic path for a dependency that depends on a runtime value, then await is helpful to load or import the messages at runtime.
```javascript
const messages = await import(`./messages-${language}.js`);
```
2. **Dependency fallbacks:**
If the imported module is failed to load, then fallback module loaded used to load the dependency.
```javascript
let lodash;
try {
lodash = await import('https://first.domain.com/lodash');
} catch {
lodash = await import('https://second.domain.com/lodash');
}
```
3. **Resource initialization:**
This feature can be used to initialize an app with Database.
```javascript
import { dbConnector} from './dbUtils.js'
//connect to database
const connection = await dbConnector.connect();
export default function(){
connection.list()
}
```
**[⬆ Back to Top](#table-of-contents)**
2. ### Class fields
ES2015 introduced classes to javascript and class properties are defined in the constructor. In the below example, the Employee class has two fields defined in the constructor.
```javascript
class Employee {
constructor() {
this.name = "John"; //public
this._age=35; //private
}
const employee = new Employee();
employee.name = "Jack";
employee._age =35; //No error
```
In the above example, the private `_age` property can be accessed and modified outside of the class. The prefix `_` is just a naming convention for private field and won't enforce any strict rule.
ES2022 introduced private and static fields to the classes.
1. **Public and private fields or methods:**
ES2022 introduced public and private properties or methods like other programming languages. These properties and methods can be defined outside the constructor and private fields prefixed with # symbol followed by variable name.
In the below example, the Employee class has been defined with private variable outside the constructor.
```javascript
class Employee {
name = "John";
#age=35;
constructor() {
}
#getAge() {
return #age
}
}
const employee = new Employee();
employee.name = "Jack";
employee.#age = 35; // Throws an error
```
2. **Static fields and methods:**
Static fields and methods are applied to the class level and they can be accessed without an instance of a class. These fields and methods declared with `static` keyword
Let's define a employee class with static field and methods declated with `static` keyword.
```javascript
class Employee{
name = "John";
static #employerName="Github"
static #getEmployerName() {
return #employerName
}
}
const employee = new Employee();
employee.emp = "Jack";
employee.#employerName = 35; // Throws an error
```
**[⬆ Back to Top](#table-of-contents)**
3. ### Array .at() method
The `.at()` method is used to access an array or string elements by passing the negative index value. i.e, It allows accessing values from the end of an array or from a string.
Before ES2022, You should have to access the elements from the end as below,
```javascript
const array = [1, 2, 3, 4, 5];
console.log(array[array.length - 2]); //4
console.log(array.slice(-2)[0]); //4
const string = '12345';
console.log(string[string.length - 2]); // '4'
console.log(string.slice(-2)[0]); // '4'
```
Now you should be able to write:
```javascript
const array = [1, 2, 3, 4, 5];
console.log(array.at(-2)); // 4
const string = '12345';
console.log(string.at(-2));
```
**[⬆ Back to Top](#table-of-contents)**
4. ### Error Cause
The `cause` property is added to the Error() constructor as an extra parameter which allow errors to be chained similar to Java-like stack traces in error chains.
In the below example, let's catch an error from JSON processing and re-throw it with a new meaningful message along with the original cause of the error.
```javascript
function processUserData(arrayData) {
return arrayData.map(data => {
try {
const json = JSON.parse(data);
return json;
} catch (err) {
throw new Error(
`Data processing failed`,
{cause: err}
);
}
});
}
```
**[⬆ Back to Top](#table-of-contents)**
5. ### hasOwn
The new `Object.hasOwn()` method is a replacement or improved version of `Object.prototype.hasOwnProperty`. It is a static method that returns true if the specified object has the indicated property as its own property. If the property is inherited, or does not exist, the method returns false.
It's not only more concise and readable but also overcomes the below limitations of `hasOwnProperty`.
1. **When `hasOwnProperty`overwritten:**
There are cases where you need to define customized `hasOwnProperty` on the object. When you try to apply `hasOwnProperty` to determine the own property or not, it throws an error as shown in below example.
```javascript
const user = {
age: 35,
hasOwnProperty: ()=> {
return false;
}
};
user.hasOwnProperty('age') // throws a TypeError
```
This issue can be solved by hasOwn method .
```javascript
const user = {
age: 35,
hasOwnProperty: ()=> {
return false;
}
};
user.hasOwn('age') // true
```
2. **Create an object with create(null) function:**
If you create new object with help of create(null) function, then newly created object doesn’t inherit from Object.prototype. So it doesn't have hasOwnProperty method.
Let's take an example to verify hasOwnProperty on an object created with create(null) function.
```javascript
const user = Object.create(null);
user.age = 35;
user.hasOwnProperty('age'); // throws a TypeError
```
In this case, hasOwn() method can be used to determine the own property.
```javascript
const user = Object.create(null);
user.age = 35;
user.hasOwn('age'); // true
```
**[⬆ Back to Top](#table-of-contents)**
6. ### Regex match indices
Regex match has been upgraded to include more information about the matching groups. The additional information includes starting and ending indices of matches in a RegExp with the usage of `\d` flag in the input string.
Let's take an example of regex pattern on the input string without `\d` flag and the information of the matches.
```javascript
const regexPatter = /Jack/g;
const input = 'Authos: Jack, Alexander and Jacky';
const result = [...input.matchAll(regexPatter)];
console.log(result[0]); // ['Jack', index: 8, input: 'Authos: Jack, Alex and Jacky', groups: undefined]
```
Whereas `\d` flag provides an additional array with the indices of the different elements that match the regex,
```javascript
const regexPatter = /(Jack)/gd;
const input = 'Authos: Jack, Alexander and Jacky';
const result = [...input.matchAll(regexPatter)];
console.log(result[0]); // ['Jack', 'Jack', index: 8, input: 'Authos: Jack, Alexander and Jacky', groups: undefined, indices: Array(2)]
```
**[⬆ Back to Top](#table-of-contents)**
## ES2023 Or ES14
ECMAScript 2023 or ES14 has been released in the month of June 2023 with some of important features like adding new methods for searching and altering the arrays, supporting symbols as keys for WeakMap API and standardizing the hashbang syntax in JavaScript.
1. ### find-array-from-last
This release introduced two new array methods named **findLast()** and **findLastIndex()** to search an array element from the last. Their functionality is similar to **find()** and **findIndex()** but searching starts from the end of an array. These methods are available on **Array** and **TypedArray** prototypes. This feature provides an efficient way for reverse order search by eliminating the process of manual array reversal.
For example, let's see the usage of finding odd elements from end of an array prior to ES2023. There are no direct methods available to search element from end of an array. Hence, the array needs to be reversed first before applying **first()** and **firstIndex()** methods.
```javascript
const isOdd = (number) => number % 2 === 1;
const numbers = [1, 2, 3, 4, 5];
const reverseNumbers = [5, 4, 3, 2, 1];
console.log(reverseNumbers.find(isOdd)); // 5
console.log(reverseNumbers.findIndex(isOdd)); // 4
```
This process is going to be simiplified in ES2023 release using **findLast()** and **findLastIndex()** methods.
```javascript
const isOdd = (number) => number % 2 === 1;
const numbers = [1, 2, 3, 4, 5];
console.log(numbers.findLast(isOdd)); // 5
console.log(numbers.findLastIndex(isOdd)); // 4
```
2. ### hashbang-syntax
Hashbang(as known as shebang) grammer has been supported with a sequence of characters(#!) at the beginning of an executable script to define the interpreter for the program to run. In other words, this syntax is helpful to tell the operating system which interpreter to use while executing the script.
For example, the below javacript file will be executed in NodeJS interpreter from Unix commandline.
```javascript
#!/usr/bin/env node
'use strict';
console.log("Hello world from hashbang syntax");
```
3. ### symbols-as-weakmap-keys
Prior to ES2023, WeakMaps are only limited to allow objects as keys because objects are unique and cannot be re-created. Since symbols are the only primitives in ECMAScript that allows unique values, WeakMap API has been extended with symbols as keys instead of just using objects.
As an example, the usage of WeakMap with objects as keys prior to ES2023 looks like below
```javascript
const weak = new WeakMap();
const objKey = { x:10 };
weak.set(objKey, "ES2023");
console.log(weak.get(objKey)); //ES2023
```
In ES2023, it is possible to use symbols as keys
```javascript
const weak = new WeakMap();
const key = Symbol("ref");
weak.set(key, "ES2023");
console.log(weak.get(key)); //ES2023
```
4. ### change-array-by-copy
Both **Array** and **TypedArray** has built-in methods such as **reverse()**, **sort()** and **splice()** to perform common actions like sorting, reverse the array elements and replacing(or removing) elements. But these methods are mutating the original array. Where as ES2023 has provided additional methods such as **toReversed()**, **toSorted()**, **toSpliced** and **with()** methods which returns new array copies instead of mutating the original array.
For example, these additional methods returns new array copies for number array without mutating the original array as shown below,
```javascript
const numbers = [1, 3, 2, 4, 5];
// toReversed
const reversedArray = numbers.toReversed();
console.log(reversedArray); // [5, 4, 2, 3, 1]
console.log(numbers); // [1, 3, 2, 4, 5]
// toSorted
const sortedArray = numbers.toSorted();
console.log(sortedArray); // [1, 2, 3, 4, 5]
console.log(numbers); // [1, 3, 2, 4, 5]
// toSpliced
const splicedArray = numbers.toSpliced(1, 3);
console.log(splicedArray); // [1, 5]
console.log(numbers); // [1, 3, 2, 4, 5]
// with
const replaceWithArray = numbers.with(2, 10);
console.log(replaceWithArray); // [1, 3, 10, 4, 5]
console.log(numbers); // [1, 3, 2, 4, 5]
```
**[⬆ Back to Top](#table-of-contents)**