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https://github.com/David-Else/modern-typescript-with-examples-cheat-sheet
Fully printable summary of modern TypeScript language features with extensive examples to help you learn
https://github.com/David-Else/modern-typescript-with-examples-cheat-sheet
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Fully printable summary of modern TypeScript language features with extensive examples to help you learn
- Host: GitHub
- URL: https://github.com/David-Else/modern-typescript-with-examples-cheat-sheet
- Owner: David-Else
- License: gpl-3.0
- Created: 2020-02-25T17:49:11.000Z (over 4 years ago)
- Default Branch: master
- Last Pushed: 2023-07-19T11:11:09.000Z (about 1 year ago)
- Last Synced: 2024-04-27T17:32:42.606Z (5 months ago)
- Topics: cheatsheet, typescript
- Language: JavaScript
- Size: 60.5 KB
- Stars: 907
- Watchers: 23
- Forks: 42
- Open Issues: 0
-
Metadata Files:
- Readme: README.md
- License: LICENSE
Awesome Lists containing this project
README
- [**Modern TypeScript with Examples Cheat Sheet**](#modern-typescript-with-examples-cheat-sheet)
- [Typing Objects](#typing-objects)
- [`Object` Versus `object`](#object-versus-object)
- [Interface Signatures Overview](#interface-signatures-overview)
- [Index Signature](#index-signature)
- [Call Signature](#call-signature)
- [Construct Signature](#construct-signature)
- [Type Literal Syntax](#type-literal-syntax)
- [Excess Properties (⛔ Inconsistency)](#excess-properties--inconsistency)
- [Mapped Types - Getting Types from Data](#mapped-types---getting-types-from-data)
- [`typeof` / `keyof` Examples](#typeof--keyof-examples)
- [`keyof` with Generics and Interfaces Example](#keyof-with-generics-and-interfaces-example)
- [Immutability](#immutability)
- [`readonly` Properties](#readonly-properties)
- [`readonly` Class Properties](#readonly-class-properties)
- [`readonly` Array / Tuple](#readonly-array--tuple)
- [`const` Assertions](#const-assertions)
- [Strict Mode](#strict-mode)
- [Non-Nullable Types `--strictNullChecks`](#non-nullable-types---strictnullchecks)
- [Strict Bind Call Apply `--strictBindCallApply`](#strict-bind-call-apply---strictbindcallapply)
- [Strict Class Property Initialization `--strictPropertyInitialization`](#strict-class-property-initialization---strictpropertyinitialization)
- [Types](#types)
- [`never`](#never)
- [`unknown`](#unknown)
- [Reading `JSON` from `localStorage` using `unknown` Example](#reading-json-from-localstorage-using-unknown-example)
- [Generics](#generics)
- [With and Without Type Argument Inference](#with-and-without-type-argument-inference)
- [Using More Than One Type Argument](#using-more-than-one-type-argument)
- [Higher Order Function with `Parameters` and `ReturnType`](#higher-order-function-with-parameterst-and-returntypet)
- [Advanced Factory using `ConstructorParameters` and `InstanceType`](#advanced-factory-using-constructorparameterst-and-instancetypet)
- [Discriminated Unions](#discriminated-unions)
- [Exhaustive Pattern Matching Using `never`](#exhaustive-pattern-matching-using-never)
- [Optional Chaining](#optional-chaining)
- [`?.` returns `undefined` when hitting a `null` or `undefined`](#-returns-undefined-when-hitting-a-null-or-undefined)
- [Nullish Coalescing](#nullish-coalescing)
- [`??` “fall Backs” to a Default Value When Dealing with `null` or `undefined`](#-fall-backs-to-a-default-value-when-dealing-with-null-or-undefined)
- [Assertion Functions](#assertion-functions)
- [A Standard JavaScript `Assert()` Doesn’t Work for Type Checking](#a-standard-javascript-assert-doesnt-work-for-type-checking)
- [Using `if` and `typeof` Everywhere is Bloat](#using-if-and-typeof-everywhere-is-bloat)
- [Assertion Function Style 1 - Check for a Condition](#assertion-function-style-1---check-for-a-condition)
- [Assertion Function Style 2 - Tell TypeScript That a Specific Variable or Property Has a Different Type](#assertion-function-style-2---tell-typescript-that-a-specific-variable-or-property-has-a-different-type)
# **Modern TypeScript with Examples Cheat Sheet**
# Typing Objects
## `Object` Versus `object`
`Object` is the type of all instances of class `Object`.
- It describes functionality that is common to all JavaScript objects
- It includes primitive values
```ts
const obj1 = {};
obj1 instanceof Object; // true
obj1.toString === Object.prototype.toString; // true
function fn(x: Object) {}
fn("foo"); // OK
```
`object` is the type of all non-primitive values.
```ts
function fn(x: object) {}
fn("foo"); // Error: "foo" is a primitive
```
## Interface Signatures Overview
```ts
interface ExampleInterface {
myProperty: boolean; // Property signature
myMethod(x: string): void; // Method signature, 'x' for documentation only
[prop: string]: any; // Index signature
(x: number): string; // Call signature
new (x: string): ExampleInstance; // Construct signature
readonly modifierOne: string; // readonly modifier
modifierTwo?: string; // optional modifier
}
```
### Index Signature
Helps to describe Arrays or objects that are used as dictionaries.
- If there are both an index signature and property and/or method signatures in
an interface, then the type of the index property value must also be a
supertype of the type of the property value and/or method
```ts
interface I1 {
[key: string]: boolean;
// 'number' is not assignable to string index type 'boolean'
myProp: number;
// '() => string' is not assignable to string index type 'boolean'
myMethod(): string;
}
interface I2 {
[key: string]: number;
myProp: number; // OK
}
```
### Call Signature
Enables interfaces to describe functions, `this` is the optional calling context
of the function in this example:
```ts
interface ClickListener {
(this: Window, e: MouseEvent): void;
}
const myListener: ClickListener = e => {
console.log("mouse clicked!", e);
};
addEventListener("click", myListener);
```
### Construct Signature
Enables describing classes and constructor functions. A class has two types:
- The type of the static side
- The type of the instance side
The constructor sits in the static side, when a class implements an interface,
only the instance side of the class is checked.
```ts
interface ClockInterface {
tick(): void;
}
interface ClockConstructor {
new (hour: number, minute: number): ClockInterface;
}
// Using Class Expression
const ClockA: ClockConstructor = class Clock implements ClockInterface {
constructor(h: number, m: number) {}
tick() {}
};
const clockClassExpression = new ClockA(18, 11);
// Using Class Declaration with a Constructor Function
class ClockB implements ClockInterface {
constructor(h: number, m: number) {}
tick() {}
}
function createClock(
ctor: ClockConstructor,
hour: number,
minute: number
): ClockInterface {
return new ctor(hour, minute);
}
const clockClassDeclaration = createClock(ClockB, 12, 17);
```
## Type Literal Syntax
Typically used in the signature of a higher-order function.
```ts
type MyFunctionType = (name: string) => number;
```
## Excess Properties (⛔ Inconsistency)
- Engineers **can’t** just think of interfaces as “objects that have exactly a
set of properties” or “objects that have at least a set of properties”.
In-line object arguments receive an additional level of validation that
doesn’t apply when they’re passed as variables.
- TypeScript is a **structurally** typed language. To create a `Dog` you don’t
need to explicitly extend the `Dog` interface, any object with a `breed`
property that is of type `string` can be used as a `Dog`:
```ts
interface Dog {
breed: string;
}
function printDog(dog: Dog) {
console.log("Dog: " + dog.breed);
}
const ginger = {
breed: "Airedale",
age: 3
};
printDog(ginger); // excess properties are OK!
printDog({
breed: "Airedale",
age: 3
});
// Excess properties are NOT OK!!
// Argument of type '{ breed: string; age: number; }' is not assignable...
```
# Mapped Types - Getting Types from Data
## `typeof` / `keyof` Examples
```ts
const data = {
value: 123,
text: "text",
subData: {
value: false
}
};
type Data = typeof data;
// type Data = {
// value: number;
// text: string;
// subData: {
// value: boolean;
// }
```
```ts
const data = ["text 1", "text 2"] as const;
type Data = typeof data[number]; // "text 1" | "text 2"
```
```ts
const locales = [
{
locale: "se",
language: "Swedish"
},
{
locale: "en",
language: "English"
}
] as const;
type Locale = typeof locales[number]["locale"]; // "se" | "en"
```
```ts
const currencySymbols = {
GBP: "£",
USD: "$",
EUR: "€"
};
type CurrencySymbol = keyof typeof currencySymbols; // "GBP" | "USD" | "EUR"
```
## `keyof` with Generics and Interfaces Example
```ts
interface HasPhoneNumber {
name: string;
phone: number;
}
interface HasEmail {
name: string;
email: string;
}
interface CommunicationMethods {
email: HasEmail;
phone: HasPhoneNumber;
fax: { fax: number };
}
function contact(
method: K,
contact: CommunicationMethods[K] // turning key into value - a mapped type
) {
//...
}
contact("email", { name: "foo", email: "[email protected]" });
contact("phone", { name: "foo", phone: 3213332222 });
contact("fax", { fax: 1231 });
// // we can get all values by mapping through all keys
type AllCommKeys = keyof CommunicationMethods;
type AllCommValues = CommunicationMethods[keyof CommunicationMethods];
```
# Immutability
## `readonly` Properties
Properties marked with `readonly` can only be assigned to during initialization
or from within a constructor of the same class.
```ts
type Point = {
readonly x: number;
readonly y: number;
};
const origin: Point = { x: 0, y: 0 }; // OK
origin.x = 100; // Error
function moveX(p: Point, offset: number): Point {
p.x += offset; // Error
return p;
}
function moveX(p: Point, offset: number): Point {
// OK
return {
x: p.x + offset,
y: p.y
};
}
```
## `readonly` Class Properties
Gettable area property is implicitly read-only because there’s no setter:
```ts
class Circle {
readonly radius: number;
constructor(radius: number) {
this.radius = radius;
}
get area() {
return Math.PI * this.radius ** 2;
}
}
```
## `readonly` Array / Tuple
```ts
const array: readonly string[];
const tuple: readonly [string, string];
```
## `const` Assertions
- `number` becomes number literal
```ts
// Type '10'
let x = 10 as const;
```
- array literals become `readonly` tuples
```ts
// Type 'readonly [10, 20]'
let y = [10, 20] as const;
```
- object literals get `readonly` properties
- no literal types in that expression should be widened (e.g. no going from
`"hello"` to `string`)
```ts
// Type '{ readonly text: "hello" }'
let z = { text: "hello" } as const;
```
- ⛔ `const` contexts **don’t** immediately convert an expression to be fully
immutable.
```ts
let arr = [1, 2, 3, 4];
let foo = {
name: "foo",
contents: arr
} as const;
foo.name = "bar"; // Error
foo.contents = []; // Error
foo.contents.push(5); // OK
```
# Strict Mode
```json
strict: true /* Enable all strict type-checking options. */
```
is equivalent to enabling all of the strict mode family options:
```json
noImplicitAny: true /* Raise error on expressions and declarations with an implied 'any' type */,
strictNullChecks: true /* Enable strict null checks */,
strictFunctionTypes: true /* Enable strict checking of function types */,
strictBindCallApply: true /* Enable strict 'bind', 'call', and 'apply' methods on functions */,
strictPropertyInitialization: true /* Enable strict checking of property initialization in classes */,
noImplicitThis: true /* Raise error on 'this' expressions with an implied 'any' type */,
alwaysStrict: true /* Parse in strict mode and emit "use strict" for each source file */
```
You can then turn off individual strict mode family checks as needed.
## Non-Nullable Types `--strictNullChecks`
In strict null checking mode, `null` and `undefined` are no longer assignable to
every type.
```ts
let name: string;
name = "Marius"; // OK
name = null; // Error
name = undefined; // Error
```
```ts
let name: string | null;
name = "Marius"; // OK
name = null; // OK
name = undefined; // Error
```
Optional parameter `?` automatically adds `| undefined`
```ts
type User = {
firstName: string;
lastName?: string; // same as `string | undefined`
};
```
- In JavaScript, every function parameter is optional, when left off their value
is `undefined`.
- We can get this functionality in TypeScript by adding a `?` to the end of
parameters we want to be optional. This is different from adding `| undefined`
which requires the parameter to be explicitly passed as `undefined`
```ts
function fn1(x: number | undefined): void {
x;
}
function fn2(x?: number): void {
x;
}
fn1(); // Error
fn2(); // OK
fn1(undefined); // OK
fn2(undefined); // OK
```
Type guard needed to check if Object is possibly `null`:
```ts
function getLength(s: string | null) {
// Error: Object is possibly 'null'.
return s.length;
}
```
```ts
function getLength(s: string | null) {
if (s === null) {
return 0;
}
return s.length;
}
// JS's truthiness semantics support type guards in conditional expressions
function getLength(s: string | null) {
return s ? s.length : 0;
}
```
```ts
function doSomething(callback?: () => void) {
// Error: Object is possibly 'undefined'.
callback();
}
```
```ts
function doSomething(callback?: () => void) {
if (typeof callback === "function") {
callback();
}
}
```
## Strict Bind Call Apply `--strictBindCallApply`
> The `call()` method calls a function with a given `this` value and arguments
> provided individually, while `apply()` accepts a single array of arguments.
> The `bind()` method creates a new function.
When set, TypeScript will check that the built-in methods of functions `call`,
`bind`, and `apply` are invoked with correct argument for the underlying
function:
```ts
function fn(x: string) {
return parseInt(x);
}
const n1 = fn.call(undefined, "10"); // OK
const n2 = fn.call(undefined, false); // `false` is not assignable to parameter of type `string`
```
## Strict Class Property Initialization `--strictPropertyInitialization`
Verify that each instance property declared in a class either:
- Has an explicit initializer, or
- Is definitely assigned to in the constructor
```ts
// Error
class User {
// 'username' has no initializer & not definitely assigned in constructor
username: string;
}
// OK
class User {
username = "n/a";
}
const user = new User();
const username = user.username.toLowerCase();
// OK
class User {
constructor(public username: string) {}
}
const user = new User("mariusschulz");
const username = user.username.toLowerCase();
```
- Has a type that includes undefined
```ts
class User {
username: string | undefined;
}
const user = new User();
// Whenever we want to use the username property as a string, we first have
// to make sure that it actually holds a string, not the value undefined
const username =
typeof user.username === "string" ? user.username.toLowerCase() : "n/a";
```
# Types
## `never`
`never` represents the type of values that never occur. It is used in the
following two places:
- As the return type of functions that never return
- As the type of variables under type guards that are never true
`never` can be used in control flow analysis:
```ts
function controlFlowAnalysisWithNever(value: string | number) {
if (typeof value === "string") {
value; // Type string
} else if (typeof value === "number") {
value; // Type number
} else {
value; // Type never
}
}
```
## `unknown`
`unknown` is the type-safe counterpart of the `any` type: we have to do some
form of checking before performing most operations on values of type `unknown`.
### Reading `JSON` from `localStorage` using `unknown` Example
```ts
type Result =
| { success: true; value: unknown }
| { success: false; error: Error };
function tryDeserializeLocalStorageItem(key: string): Result {
const item = localStorage.getItem(key);
if (item === null) {
// The item does not exist, thus return an error result
return {
success: false,
error: new Error(`Item with key "${key}" does not exist`)
};
}
let value: unknown;
try {
value = JSON.parse(item);
} catch (error) {
// The item is not valid JSON, thus return an error result
return {
success: false,
error
};
}
// Everything's fine, thus return a success result
return {
success: true,
value
};
}
```
# Generics
## With and Without Type Argument Inference
```ts
function identity(arg: T): T {
return arg;
}
let output = identity("myString"); // Type of output is 'string'
let output = identity("myString"); // The compiler sets the value of `T`
```
## Using More Than One Type Argument
```ts
function makePair() {
let pair: { first: F; second: S };
function getPair() {
return pair;
}
function setPair(x: F, y: S) {
pair = {
first: x,
second: y
};
}
return { getPair, setPair };
}
const { getPair, setPair } = makePair(); // Creates a pair
setPair(1, "y"); // Must pass (number, string)
```
## Higher Order Function with `Parameters` and `ReturnType`
```ts
function logDuration any>(func: T) {
const funcName = func.name;
// Return a new function that tracks how long the original took
return (...args: Parameters): ReturnType => {
console.time(funcName);
const results = func(...args);
console.timeEnd(funcName);
return results;
};
}
function addNumbers(a: number, b: number): number {
return a + b;
}
// Hover over is `addNumbersWithLogging: (a: number, b: number) => number`
const addNumbersWithLogging = logDuration(addNumbers);
addNumbersWithLogging(5, 3);
```
## Advanced Factory using `ConstructorParameters` and `InstanceType`
```ts
class Hero {
constructor(public point: [number, number]) {}
}
const entities = [];
const entityFactory = <
T extends {
new (...args: any[]): any;
}
>(
classToCreate: T,
numberOf: number,
...args: ConstructorParameters
): InstanceType[] =>
[...Array(numberOf)].map(() => new classToCreate(...args));
entities.push(...entityFactory(Hero, 10, [12, 10]));
```
# Discriminated Unions
A data structure used to hold a value that could take on several different, but
fixed, types.
## Exhaustive Pattern Matching Using `never`
```ts
interface Square {
kind: "square";
size: number;
}
interface Rectangle {
kind: "rectangle";
width: number;
height: number;
}
interface Circle {
kind: "circle";
radius: number;
}
interface Triangle {
kind: "triangle";
whatever: number;
}
type Shape = Square | Rectangle | Circle | Triangle;
function assertNever(x: never): never {
throw new Error("Unexpected object: " + x);
}
function area(s: Shape) {
switch (s.kind) {
case "square":
return s.size * s.size;
case "rectangle":
return s.height * s.width;
case "circle":
return Math.PI * s.radius ** 2;
default:
return assertNever(s); // Error
// Argument of type 'Triangle' not assignable to param of type 'never'
}
}
```
# Optional Chaining
## `?.` returns `undefined` when hitting a `null` or `undefined`
Album where the artist, and the artists biography might not be present in the
data.
```ts
type AlbumAPIResponse = {
title: string;
artist?: {
name: string;
bio?: string;
previousAlbums?: string[];
};
};
// Instead of:
const maybeArtistBio = album.artist && album.artist.bio;
// ?. acts differently than && on "falsy" values: empty string, 0, NaN, false
const artistBio = album?.artist?.bio;
// optional chaining also works with the [] operators when accessing elements
const maybeArtistBioElement = album?.["artist"]?.["bio"];
const maybeFirstPreviousAlbum = album?.artist?.previousAlbums?.[0];
```
Optional chaining on an optional function:
```ts
interface OptionalFunction {
bar?: () => number;
}
const foo: OptionalFunction = {};
const bat = foo.bar?.(); // number | undefined
```
# Nullish Coalescing
## `??` “fall Backs” to a Default Value When Dealing with `null` or `undefined`
Value `foo` will be used when it’s “present”; but when it’s `null` or
`undefined`, calculate `bar()` in its place.
```ts
let x = foo ?? bar();
// instead of
let x = foo !== null && foo !== undefined ? foo : bar();
```
It can replace uses of `||` when trying to use a default value, and avoids bugs.
When `localStorage.volume` is set to `0`, the page will set the volume to `0.5`
which is unintended. `??` avoids some unintended behaviour from `0`, `NaN` and
`""` being treated as falsy values.
```ts
function initializeAudio() {
const volume = localStorage.volume || 0.5; // Potential bug
}
```
# Assertion Functions
Assertions in JavaScript are often used to guard against **improper** types
being passed in.
## A Standard JavaScript `Assert()` Doesn’t Work for Type Checking
```ts
function yell(str) {
assert(typeof str === "string");
return str.toUppercase(); // Oops! We misspelled 'toUpperCase'
}
```
## Using `if` and `typeof` Everywhere is Bloat
```ts
function yell(str) {
if (typeof str !== "string") {
throw new TypeError("str should have been a string.");
}
// Error caught!
return str.toUppercase();
}
```
## Assertion Function Style 1 - Check for a Condition
```ts
function assert(condition: any, msg?: string): asserts condition {
if (!condition) {
throw new AssertionError(msg);
}
}
function yell(str) {
assert(typeof str === "string");
return str.toUppercase();
// ~~~~~~~~~~~
// error: Property 'toUppercase' does not exist on type 'string'.
// Did you mean 'toUpperCase'?
}
```
## Assertion Function Style 2 - Tell TypeScript That a Specific Variable or Property Has a Different Type
Very similar to writing type predicate signatures.
```ts
function assertIsString(val: any): asserts val is string {
if (typeof val !== "string") {
throw new AssertionError("Not a string!");
}
}
function yell(str: any) {
assertIsString(str);
// Now TypeScript knows that 'str' is a 'string'.
return str.toUppercase();
// ~~~~~~~~~~~
// error: Property 'toUppercase' does not exist on type 'string'.
// Did you mean 'toUpperCase'?
}
```
#### Thanks to the following sites and people for providing many of the fantastic examples:
[typescriptlang.org](https://www.typescriptlang.org/docs/home.html)
[Marius Schulz - Blog](https://mariusschulz.com/)
[Mike North - TypeScript 3 Fundamentals v2](https://frontendmasters.com/courses/typescript-v2/)
[Shu Uesugi - TypeScript for Beginner Programmers](https://ts.chibicode.com/)
[Dr. Axel Rauschmayer - 2ality](https://2ality.com/index.html)