What does this symbol mean in JavaScript?

Question

question from:https://stackoverflow.com/questions/65930411/how-to-add-multiple-conditions-in-an-if-statement

Answer 1

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

• (...args) => {}?—?What is the meaning of “…args” (three dots) in a function definition?
• [...iter]?—?javascript es6 array feature […data, 0] “spread operator”
• {...props}?—?Javascript Property with three dots (…)

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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?
• <a href="https://stackoverflow.com/q/313