What Does the at Symbol (@) Do in Es6 JavaScript? (Ecmascript 2015)

What does the at symbol (@) do in ES6 javascript? (ECMAScript 2015)

It's a decorator. It's a proposal to be added to ECMAScript. There are multiple ES6 and ES5 equivalent examples on: javascript-decorators.

Decorators dynamically alter the functionality of a function, method, or class without having to directly use subclasses or change the source code of the function being decorated.

They are commonly used to control access, registration, annotation.

What does @@ (at at) mean in ES6 JavaScript?

@@ describes what's called a well-known symbol. (Note that it isn't actually valid syntax in JS.) According to the ES6/ES2015¹ specification:

Well-known symbols are built-in Symbol values that are explicitly referenced by algorithms of this specification. They are typically used as the keys of properties whose values serve as extension points of a specification algorithm. Unless otherwise specified, well-known symbols values are shared by all Code Realms (8.2).

Code Realms refer to different instances of a JavaScript environment. For example, the Code Realm of the root document would be different to that of JavaScript running in an <iframe>.

An example of where it matter what code realm an object comes from is when trying to use instanceof to determine whether an object is an array (hint: it won't work if it's from another frame). To avoid these kinds of issues from popping up with symbols, they are shared so that references to (say) @@toString will work no matter where the object came from.

Some of these are exposed directly through the Symbol constructor, for example, @@toPrimitive is exposed as Symbol.toPrimitive. That can be used to override the value produced when attempting to convert an object to a primitive value, for example:

let a = { [Symbol.toPrimitive]: () => 1 };
console.log(+a); // 1
console.log(a.valueOf()); // (the same object)
console.log(a.toString()); // "[object Object]"

In general, symbols are used to provide unique properties on an object which cannot collide with a random property name, for example:

let a = Symbol();
let foo = { [a]：　1　};
foo[a]; // 1

There is no way to access the value except by getting the symbol from somewhere (though you can get all symbols for an object by calling Object.getOwnPropertySymbols, so they cannot be used to implement private properties or methods).

^{^1: See this es-discuss topic for some discussion about the different names.}

What is valid syntax of and what are uses of @ at javascript?

It is called a decorator.

https://github.com/wycats/javascript-decorators

Medium - Exploring es7 decorators

What is the at symbol (@) doing in React with ES6 and mixins?

Ok, seems this is coming from a ES7 decorator proposal used in https://github.com/brigand/react-mixin.

We are using Babel in our webpack loader with the stage: 1 config set, allowing Babel to compile that decorator syntax. This gives the class access, via a wrapper, to the methods of the mixin.

If anyone can explain this more thoroughly, feel free!

What is the use of Symbol in javascript ECMAScript 6?

Symbol is used to create a totally unique, one-of-a-kind identifier. It's use is precisely for the example you list.

Even if you call Symbol with the same string, the instances will be different. This allows different libraries (which may be used at the same time) to define keys which may be used at the same time.

For example, imagine two libraries using a common name to define something on window or global (or for illustration, the fake global door):

const door = {};// from library 1door.cake = () => console.log('chocolate');// from library 2door.cake = () => console.log('vanilla');
// your codedoor.cake();

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, 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.

`{`…`}`, `{ 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 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

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?

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?

Conditional operator

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

Question mark and colon in JavaScript
Operator precedence with JavaScript's ternary operator
How do you use the ? : (conditional) operator in JavaScript?

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

`<<=`, `>>=`, `>>>=`, `&=`, `^=`, `|=` — Double less than, double greater than, triple greater than, ampersand, caret, or pipe followed by equal sign: bitwise assignments

What do these JavaScript bitwise operators do?

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

Comma operator

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

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

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 is the difference between "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

Label

`label:` — Colon: labels

What does the JavaScript syntax foo: mean?
What does ':' (colon) do in JavaScript?

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?