Symbol

Baseline *

20159

Symbol

Symbol() Symbol.for("key") "key" Symbol Symbol.for("key") Symbol Symbol Symbol Symbol Symbol

Symbol()

const sym1 = Symbol();
const sym2 = Symbol("foo");
const sym3 = Symbol("foo");

3 Symbol("foo") "foo"

Symbol("foo") === Symbol("foo"); // false

new TypeError

const sym = new Symbol(); // TypeError

Symbol new Booleannew Stringnew Number

Symbol Object()

const sym = Symbol("foo");
typeof sym; // "symbol"
const symObj = Object(sym);
typeof symObj; // "object"

2 WeakMapWeakSetWeakRefFinalizationRegistry

Symbol() Symbol.for() Symbol.keyFor()

JavaScript for() keyFor() JavaScript

Symbol.for(tokenString) Symbol.keyFor(symbolValue) true

Symbol.keyFor(Symbol.for("tokenString")) === "tokenString"; // true

WeakMap, WeakSet, WeakRef, FinalizationRegistry

Symbol JavaScript Symbol.hasInstance instanceof

JavaScript JSON.stringify toJSON() String toString() valueOf()

: @@<> Symbol.hasInstance @@hasInstance Array.prototype[Symbol.iterator]() Array.prototype[Symbol.iterator]()

Array.prototype WeakMapWeakSetWeakRefFinalizationRegistry

Object.getOwnPropertySymbols()

Symbol(): new

Symbol.hasInstance ...

Symbol.asyncDispose: await using
Symbol.asyncIterator: (AsyncIterator) for await...of
Symbol.dispose: using
Symbol.hasInstance: instanceof
Symbol.isConcatSpreadable: Array.prototype.concat()
Symbol.iterator: for...of
Symbol.match: String.prototype.match()
Symbol.matchAll: String.prototype.matchAll()
Symbol.replace: String.prototype.replace()
Symbol.search: String.prototype.search()
Symbol.species
Symbol.split: String.prototype.split()
Symbol.toPrimitive
Symbol.toStringTag: Object.prototype.toString()
Symbol.unscopables: with

Symbol.for(): key key
Symbol.keyFor()

Symbol.prototype Symbol

Symbol.prototype.constructor: Symbol Symbol
Symbol.prototype.description
Symbol.prototype[Symbol.toStringTag]: [Symbol.toStringTag] "Symbol" Object.prototype.toString() Symbol toString() thisArg Object.prototype.toString.call()

Symbol.prototype.toString(): Object.prototype.toString()
Symbol.prototype.valueOf(): Object.prototype.valueOf()
Symbol.prototype[Symbol.toPrimitive]()

typeof

typeof

typeof Symbol() === "symbol";
typeof Symbol("foo") === "symbol";
typeof Symbol.iterator === "symbol";

TypeError : +sym sym | 0
Object(sym) == sym true
Symbol("foo") + "bar" TypeError
String(sym) Symbol.prototype.toString() new String(sym)

for...in

for...in Object.getOwnPropertyNames() Object.getOwnPropertySymbols()

const obj = {};

obj[Symbol("a")] = "a";
obj[Symbol.for("b")] = "b";
obj["c"] = "c";
obj.d = "d";

for (const i in obj) {
  console.log(i);
}
// "c" "d"

JSON.stringify()

JSON.stringify()

JSON.stringify({ [Symbol("foo")]: "foo" });
// '{}'

JSON.stringify()

Symbol

const sym = Symbol("foo");
const obj = { [sym]: 1 };
obj[sym]; // 1
obj[Object(sym)]; // still 1


ECMAScript 2027 LanguageSpecification # sec-symbol-objects

Symbol (core-js)
typeof
JavaScript
ES6 In Depth: Symbols (hacks.mozilla.org, 2015)

MDN

MDN 2026523

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