https://github.com/ferreiratiago/js-wtf
Some of the weirdest things on JavaScript
https://github.com/ferreiratiago/js-wtf
array destructuring-assignment iife javascript lexical-grammar math number operators statistics string temporary-dead-zones wtf
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Some of the weirdest things on JavaScript
- Host: GitHub
- URL: https://github.com/ferreiratiago/js-wtf
- Owner: ferreiratiago
- Created: 2017-05-11T17:28:47.000Z (about 8 years ago)
- Default Branch: master
- Last Pushed: 2017-05-21T19:00:48.000Z (about 8 years ago)
- Last Synced: 2023-09-28T13:57:49.992Z (over 1 year ago)
- Topics: array, destructuring-assignment, iife, javascript, lexical-grammar, math, number, operators, statistics, string, temporary-dead-zones, wtf
- Homepage:
- Size: 26.4 KB
- Stars: 2
- Watchers: 1
- Forks: 0
- Open Issues: 0
-
Metadata Files:
- Readme: README.md
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README
# What the f** JavaScript
These are some of the weirdest things brought to us by JavaScript.
* [Number](#number)
* [Math](#math)
* [Operators](#operators)
* [String](#string)
* [Array](#array)
* [Immediately Invoked Function Expression](#immediately-invoked-function-expression)
* [Lexical Grammar](#lexical-grammar)
* [Statements](#statements)
* [Destructuring](#destructuring)
* [Temporary Dead Zones](#temporary-dead-zones)
## [Number](https://github.com/ferreiratiago/js-wtf/blob/master/number.md)
```js
Number('0.') // 0
Number('.0') // 0
Number('.') // NaN
```
[*Why?*](https://github.com/ferreiratiago/js-wtf/blob/master/number.md#wtf)
```js
Number({}) // NaN
Number([]) // 0
```
[*Why?*](https://github.com/ferreiratiago/js-wtf/blob/master/number.md#wtf-1)
```js
Number(undefined) // NaN
Number(null) // 0
```
[*Why?*](https://github.com/ferreiratiago/js-wtf/blob/master/number.md#wtf-2)
```js
Number('0O0') // 0
Number('0X0') // 0
```
[*Why?*](https://github.com/ferreiratiago/js-wtf/blob/master/number.md#wtf-3)
```js
Number.MAX_VALUE > 0; //true
Number.MIN_VALUE < 0; //false
```
[*Why?*](https://github.com/ferreiratiago/js-wtf/blob/master/number.md#wtf-4)
```js
42.toFixed(2) // SyntaxError: Invalid or unexpected token
42. toFixed(2) // SyntaxError: Unexpected identifier
42 .toFixed(2) // '42.00'
42 . toFixed(2) // '42.00'
42.0.toFixed(2) // '42.00'
42..toFixed(2) // '42.00'
(42).toFixed(2) // '42.00'
```
[*Why?*](https://github.com/ferreiratiago/js-wtf/blob/master/number.md#why-5)
## [Math](https://github.com/ferreiratiago/js-wtf/blob/master/math.md)
```js
Math.max() > Math.min() // false
```
[*Why?*](https://github.com/ferreiratiago/js-wtf/blob/master/math.md#wtf)
## [Operators](https://github.com/ferreiratiago/js-wtf/blob/master/operators.md)
```js
[] + {} // '[object Object]'
{} + [] // 0
```
[*Why?*](https://github.com/ferreiratiago/js-wtf/blob/master/operators.md#wtf)
```js
true + true === 2 // true
true - true === 0 // true
true === 1 // false
```
[*Why?*](https://github.com/ferreiratiago/js-wtf/blob/master/operators.md#wtf-1)
```js
0.1 + 0.2 === 0.3 // false
```
[*Why?*](https://github.com/ferreiratiago/js-wtf/blob/master/operators.md#wtf-2)
```js
1 < 2 < 3 // true
3 > 2 > 1 // false
```
[*Why?*](https://github.com/ferreiratiago/js-wtf/blob/master/operators.md#wtf-3)
```js
'5' + 3 // '53'
'5' - 3 // 2
```
[*Why?*](https://github.com/ferreiratiago/js-wtf/blob/master/operators.md#wtf-4)
## [String](https://github.com/ferreiratiago/js-wtf/blob/master/string.md)
```js
String(-0) // '0'
Number('-0') // -0
```
[*Why?*](https://github.com/ferreiratiago/js-wtf/blob/master/string.md#wtf)
```js
String(null) // 'null'
String([null]) // ''
String(undefined) // 'undefined'
String([undefined]) // ''
```
[*Why?*](https://github.com/ferreiratiago/js-wtf/blob/master/string.md#wtf-1)
```js
String({}) // '[object Object]'
String([]) // ''
```
[*Why?*](https://github.com/ferreiratiago/js-wtf/blob/master/string.md#wtf-2)
```js
var s = Symbol('Hello World') // Symbol(Hello World)
String(s) // 'Symbol(Hello World)'
s + '' // TypeError: Cannot convert a Symbol value to a string
```
[*Why?*](https://github.com/ferreiratiago/js-wtf/blob/master/string.md#wtf-3)
```js
parseInt('15') // 15
parseInt('15 and more') // 15
parseInt('more and 15') // NaN
```
[*Why?*](https://github.com/ferreiratiago/js-wtf/blob/master/string.md#wtf-4)
```js
(!+[]+[]+![]).length // 9
```
[*Why?*](https://github.com/ferreiratiago/js-wtf/blob/master/string.md#wtf-5)
## [Array](https://github.com/ferreiratiago/js-wtf/blob/master/array.md)
```js
[] == ![] // true
```
[*Why?*](https://github.com/ferreiratiago/js-wtf/blob/master/array.md#wtf)
```js
Array.apply(null, Array(3)) // [ undefined, undefined, undefined ]
Array.apply(null, [,,,]) // [ undefined, undefined, undefined ]
Array.apply(null, {length : 3}) // [ undefined, undefined, undefined ]
Array.apply(null, (a,b,c) => {}) // [ undefined, undefined, undefined ]
```
[*Why?*](https://github.com/ferreiratiago/js-wtf/blob/master/array.md#wtf-1)
```js
[10, 9, 8, 3, 2, 1, 0].sort() // [ 0, 1, 10, 2, 3, 8, 9 ]
```
[*Why?*](https://github.com/ferreiratiago/js-wtf/blob/master/array.md#wtf-2)
## [Immediately Invoked Function Expression](https://github.com/ferreiratiago/js-wtf/blob/master/iife.md)
```js
function () { console.log('Hello World') }() // SyntaxError: Unexpected token (
```
[*Why?*](https://github.com/ferreiratiago/js-wtf/blob/master/iife.md#wtf)
```js
function foo() { console.log('Hello World') }() // SyntaxError: Unexpected token )
```
[*Why?*](https://github.com/ferreiratiago/js-wtf/blob/master/iife.md#wtf-1)
```js
!function () { console.log('Hello World') }() // Hello World
```
[*Why?*](https://github.com/ferreiratiago/js-wtf/blob/master/iife.md#wtf-2)
## [Lexical Grammar](https://github.com/ferreiratiago/js-wtf/blob/master/lexical-grammar.md)
```js
function foo() {
return
'Hello World';
}
foo() // undefined
```
[*Why?*](https://github.com/ferreiratiago/js-wtf/blob/master/lexical-grammar.md#lexical-grammar)
## [Statements](https://github.com/ferreiratiago/js-wtf/blob/master/statements.md)
```js
switch (42) {
default:
console.log('foo');
case 10:
case 20:
console.log('bar');
break;
case 30:
console.log('zed');
break;
}
// foo
// bar
```
[*Why?*](https://github.com/ferreiratiago/js-wtf/blob/master/statements.md#wtf)
```js
function foo() {
try {
return 2
} finally {
return 3
}
}
foo() // 3
```
[*Why?*](https://github.com/ferreiratiago/js-wtf/blob/master/statements.md#wtf-1)
## [Destructuring](https://github.com/ferreiratiago/js-wtf/blob/master/destructuring.md)
```js
function foo(x = {y: 10}, {y = 20} = {}) {
console.log(x.y, y);
}
foo(); // 10 20
foo({y: 30}, {y: 40}); // 30 40
foo({}, {}); // undefined 20
```
[*Why?*](https://github.com/ferreiratiago/js-wtf/blob/master/destructuring.md#wtf)
## [Temporary Dead Zones](https://github.com/ferreiratiago/js-wtf/blob/master/tdz.md)
```js
(function(a = b, b) {
}(undefined, 1)); // ReferenceError
```
[*Why?*](https://github.com/ferreiratiago/js-wtf/blob/master/tdz.md#wtf)
```js
let b = 1;
(function(a = b, b) {
console.log(a, b);
}(undefined, 2)); // ReferenceError
```
[*Why?*](https://github.com/ferreiratiago/js-wtf/blob/master/tdz.md#wtf-1)
```js
{
typeof a; // undefined
typeof b; // ReferenceError
let b;
}
```
[*Why?*](https://github.com/ferreiratiago/js-wtf/blob/master/tdz.md#wtf-2)