What does this symbol mean in JavaScript?

Question

What is this? This is a collection of questions that come up every now and then about syntax in JavaScript. This is also a Community Wiki, so everyone is invited to participate in maintaining this list. Why is this? Stack Overflow does not allow searching for particular characters. As a consequence, many questions about operators and other syntax tokens are not found easily when searching for them. This also makes closing duplicates more difficult. The list below is to help with this issue. The main idea is to have links to existing questions on Stack Overflow, so it's easier for us to reference them, not to copy over content from the ECMAScript Spec. Additionally, this is a blatant copy of the PHP symbol reference. We needed a JS one. Please help. Edit and add links to other operators/syntax references, or if you can't find good questions/answers on a particular piece of syntax, add an answer to this question and link it

Accepted Answer

See the documentation on MDN about expressions and operators and statements.

Basic keywords and general expressions

`this` keyword:

- How does the "this" keyword work, and when should it be used?

`var x = function()` vs. `function x()` — Function declaration syntax

- var functionName = function() {} vs function functionName() {}

`(function(){`…`})()` — IIFE (Immediately Invoked Function Expression)

- What is the purpose?, How is it called?

- Why does `(function(){…})();` work but `function(){…}();` doesn't?

- `(function(){…})();` vs `(function(){…}());`

- shorter alternatives:

- `!function(){…}();` - What does the exclamation mark do before the function?

- `+function(){…}();` - JavaScript plus sign in front of function expression

- !function(){ }() vs (function(){ })(), `!` vs leading semicolon

- `(function(window, undefined){…}(window));`

`someFunction()()` — Functions which return other functions

- Two sets of parentheses after function call

`=>` — Equal sign, greater than: arrow function expression syntax

- What's the meaning of "=>" (an arrow formed from equals & greater than) in JavaScript?

`|>` — Pipe, greater than: Pipeline operator

- What does the "|>" operator do in JavaScript?

`function*`, `yield`, `yield*` — Star after `function` or `yield`: generator functions

- What is "function*" in JavaScript?

- What's the yield keyword in JavaScript?

- Delegated yield (yield star, yield *) in generator functions

`[`…`]`, `[ value ]`, `Array()` — Square brackets: array notation

- What’s the difference between "Array()" and "[]" while declaring a JavaScript array?

- What is array literal notation in javascript and when should you use it?

If the square brackets appear on the left side of an assignment (`[a] = ...`), or inside a function's parameters, it's a destructuring assignment.

- Where can I get info on the object parameter syntax for JavaScript functions?

`{`…`}`, `{ key: value }`, `{ [key]: value }` — Curly brackets: object literal syntax (not to be confused with blocks)

- What do curly braces in JavaScript mean?

- Javascript object literal: what exactly is {a, b, c}?

- What do square brackets around a property name in an object literal mean?

- How does this object method definition work without the "function" keyword? (ES2015 Method definitions)

If the curly brackets appear on the left side of an assignment (`{ a } = ...`) or inside a function's parameters, it's a destructuring assignment.

```…`${`…`}`…``` — Backticks, dollar sign with curly brackets: template literals

- What does this ``…${…}…`` code from the node docs mean?

- Usage of the backtick character (`) in JavaScript?

- What is the purpose of template literals (backticks) following a function in ES6?

`/`…`/` — Slashes: regular expression literals

- Meaning of javascript text between two slashes

`$` — Dollar sign in regex replace patterns: `$$`, `$&`, `$``, `$'`, `$n`

- JavaScript replace() method dollar signs

`()` — Parentheses: grouping operator

- MDN: Grouping operator

Property-related expressions

`obj.prop`, `obj[prop]`, `obj["prop"]` — Square brackets or dot: property accessors

- JavaScript property access: dot notation vs. brackets?

`?.`, `?.[]`, `?.()` — Question mark, dot: optional chaining operator

- Question mark after parameter

- Null-safe property access (and conditional assignment) in ES6/2015

- Optional Chaining in JavaScript

- Is there a null-coalescing (Elvis) operator or safe navigation operator in javascript?

- Is there a "null coalescing" operator in JavaScript?

`::` — Double colon: bind operator

- JavaScript double colon (bind operator)

`new` operator

- What is the 'new' keyword in JavaScript?

- What is "new.target"?

`...iter` — Three dots: spread syntax; rest parameters

- `(...rest) => {}` — What is the meaning of “…args” (three dots) in a function definition?

- `fn(...args)` — What is the meaning of “foo(…arg)” (three dots in a function call)?

- `[...iter]` — javascript es6 array feature […data, 0] “spread operator”

- `{...props}` — Javascript Property with three dots (…), What does the '…rest' stand for in this object destructuring?

Increment and decrement

`++`, `--` — Double plus or minus: pre- / post-increment / -decrement operators

- `++someVariable` vs `someVariable++` in Javascript

Unary and binary (arithmetic, logical, bitwise) operators

`delete` operator

- What is the purpose of the delete operator in Javascript?

`void` operator

- What does `void 0` mean?

`+`, `-` — Plus and minus: addition or concatenation, and subtraction operators; unary sign operators

- What does = +_ mean in JavaScript, Single plus operator in javascript

- What's the significant use of unary plus and minus operators?

- Why is [1,2] + [3,4] = "1,23,4" in JavaScript?

- Why does JavaScript handle the plus and minus operators between strings and numbers differently?

`|`, `&`, `^`, `~` — Single pipe, ampersand, circumflex, tilde: bitwise OR, AND, XOR, & NOT operators

-…

What does this symbol mean in JavaScript?

Solution

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