What does this symbol mean in JavaScript?

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

Basic keywords and general expressions

this keyword:

• How does the “this” keyword work?

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

[], 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.

{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?

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

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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?

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Increment and decrement

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

• ++someVariable vs someVariable++ in Javascript

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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 do these JavaScript bitwise operators do?
• How to: The ~ operator?
• Is there a & logical operator in Javascript
• What does the “|” (single pipe) do in JavaScript?
• What does the operator |= do in JavaScript?
• What does the ^ (caret) symbol do in JavaScript?
• Using bitwise OR 0 to floor a number, How does x|0 floor the number in JavaScript?
• Why does ~1 equal -2?
• What does ~~ (“double tilde”) do in Javascript?
• How does !!~ (not not tilde/bang bang tilde) alter the result of a ‘contains/included’ Array method call? (also here and here)

%  —  Percent sign: remainder operator

• What does % do in JavaScript?

&&, ||, !  —  Double ampersand, double pipe, exclamation point: logical operators

• Logical operators in JavaScript — how do you use them?
• Logical operator || in javascript, 0 stands for Boolean false?
• What does “var FOO = FOO || {}” (assign a variable or an empty object to that variable) mean in Javascript?, JavaScript OR (||) variable assignment explanation, What does the construct x = x || y mean?
• Javascript AND operator within assignment
• What is “x && foo()”? (also here and here)
• What is the !! (not not) operator in JavaScript?
• What is an exclamation point in JavaScript?

??  —  Double question mark: nullish-coalescing operator

• How is the nullish coalescing operator (??) different from the logical OR operator (||) in ECMAScript?
• Is there a null-coalescing (Elvis) operator or safe navigation operator in javascript?
• Is there a “null coalescing” operator in JavaScript?

**  —  Double star: power operator (exponentiation)

• x ** 2 is equivalent to Math.pow(x, 2)
• Is the double asterisk ** a valid JavaScript operator?
• MDN documentation

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Equality operators

==, ===  —  Equal signs: equality operators

• Which equals operator (== vs ===) should be used in JavaScript comparisons?
• How does JS type coercion work?
• In Javascript, <int-value> == “<int-value>” evaluates to true. Why is it so?
• [] == ![] evaluates to true
• Why does “undefined equals false” return false?
• Why does !new Boolean(false) equals false in JavaScript?
• Javascript 0 == ‘0’. Explain this example
• Why false == “false” is false?

!=, !==  —  Exclamation point and equal signs: inequality operators

• != vs. !==
• What is the difference between != and !== operators in JavaScript?

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Bit shift operators

<<, >>, >>>  —  Two or three angle brackets: bit shift operators

• What do these JavaScript bitwise operators do?
• Double more-than symbol in JavaScript
• What is the JavaScript >>> operator and how do you use it?

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Conditional operator

…?…:…  —  Question mark and colon: conditional (ternary) operator

• Question mark and colon in JavaScript
• Operator precedence with Javascript Ternary operator
• How do you use the ? : (conditional) operator in JavaScript?

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Assignment operators

=  —  Equal sign: assignment operator

• What is the difference between the `=` and `==` operators and what is `===`? (Single, double, and triple equals)

This symbol is also used for default parameters or default values in a destructuring assignment:

• what does (state = {}) => state means
• What does ({“key”: “value”} = {}) syntax mean inside a JavaScript function

%=  —  Percent equals: remainder assignment

• Having Confusion with Modulo operator

+=  —  Plus equals: addition assignment operator

• How does += (plus equal) work?

&&=, ||=, ??=  —  Double ampersand, pipe, or question mark, followed by equal sign: logical assignments

• What purpose do &&=, ||= and ??= serve?
• Replace a value if null or undefined in JavaScript
• Set a variable if undefined
• Ruby’s ||= (or equals) in JavaScript?
• Original proposal
• Specification

Destructuring

• of function parameters: Where can I get info on the object parameter syntax for JavaScript functions?
• of arrays: Multiple assignment in javascript? What does [a,b,c] = [1, 2, 3]; mean?
• of objects/imports: Javascript object bracket notation ({ Navigation } =) on left side of assign

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Comma operator

,  —  Comma operator (not to be confused with the comma used in variable declarations)

• What does a comma do in JavaScript expressions?
• Comma operator returns first value instead of second in argument list?
• When is the comma operator useful?

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Control flow

{…}  — Curly brackets: blocks (not to be confused with object literal syntax)

• JavaScript curly braces with no function or json

Declarations

var, let, const  —  Declaring variables

• What’s the difference between using “let” and “var”?
• Are there constants in JavaScript?
• What is the temporal dead zone?
• var a, b;  —  Comma used in variable declarations (not to be confused with the comma operator): JavaScript variable definition: Commas vs. Semicolons

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Label

label:  —  Colon: labels

• What does the JavaScript syntax foo: mean?
• What does ‘:’ (colon) do in JavaScript?

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Other

123n  —  n after integer: BigInt

• What does character ‘n’ after numeric literal mean in JavaScript?

#  —  Hash (number sign): Private methods or private fields

• What does the # symbol do in JavaScript?

_  —  Underscore: separator in numeric literals

• Javascript numeric separators?
• Is there a Javascript equivalent to the Ruby syntax using underscores (e.g. 10_000 = 10000) to make larger integers human readable?