Web/JavaScript/Reference/Global objects/Symbol

The data type symbol is a primitive data type. The Symbol() function returns a value of type symbol, has static properties that expose several members of built-in objects, has static methods that expose the global symbol registry, and resembles a built-in object class, but is incomplete as a constructor because it does not support the syntax "new Symbol()".

Every symbol value returned from Symbol() is unique. A symbol value may be used as an identifier for object properties; this is the data type's primary purpose, although other use-cases exist, such as enabling opaque data types, or serving as an implementation-supported unique identifier in general. Some further explanation about purpose and usage can be found in the glossary entry for Symbol.

Description

To create a new primitive symbol, you write Symbol() with an optional string as its description:

let sym1 = Symbol()
let sym2 = Symbol('foo')
let sym3 = Symbol('foo')

The above code creates three new symbols. Note that Symbol("foo") does not coerce the string "foo" into a symbol. It creates a new symbol each time:

Symbol('foo') === Symbol('foo')  // false

The following syntax with the new operator will throw a TypeError:

let sym = new Symbol()  // TypeError

This prevents authors from creating an explicit Symbol wrapper object instead of a new symbol value and might be surprising as creating explicit wrapper objects around primitive data types is generally possible (for example, new Boolean, new String and new Number).

If you really want to create a Symbol wrapper object, you can use the Object() function:

let sym = Symbol('foo')
typeof sym      // "symbol" 
let symObj = Object(sym)
typeof symObj   // "object"

Shared symbols in the global symbol registry

The above syntax using the Symbol() function will not create a global symbol that is available in your whole codebase. To create symbols available across files and even across realms (each of which has its own global scope), use the methods Symbol.for() and Symbol.keyFor() to set and retrieve symbols from the global symbol registry.

Finding symbol properties on objects

The method Object.getOwnPropertySymbols() returns an array of symbols and lets you find symbol properties on a given object. Note that every object is initialized with no own symbol properties, so that this array will be empty unless you've set symbol properties on the object.

Constructor

Symbol(): Creates a new Symbol object. It is incomplete as a constructor because it does not support the syntax "new Symbol()".

Static properties

Symbol.asyncIterator: A method that returns the default AsyncIterator for an object. Used by for await...of.
Symbol.hasInstance: A method determining if a constructor object recognizes an object as its instance. Used by instanceof.
Symbol.isConcatSpreadable: A Boolean value indicating if an object should be flattened to its array elements. Used by Array.prototype.concat().
Symbol.iterator: A method returning the default iterator for an object. Used by for...of.
Symbol.match: A method that matches against a string, also used to determine if an object may be used as a regular expression. Used by String.prototype.match().
Symbol.matchAll: A method that returns an iterator, that yields matches of the regular expression against a string. Used by String.prototype.matchAll().
Symbol.replace: A method that replaces matched substrings of a string. Used by String.prototype.replace().
Symbol.search: A method that returns the index within a string that matches the regular expression. Used by String.prototype.search().
Symbol.split: A method that splits a string at the indices that match a regular expression. Used by String.prototype.split().
Symbol.species: A constructor function that is used to create derived objects.
Symbol.toPrimitive: A method converting an object to a primitive value.
Symbol.toStringTag: A string value used for the default description of an object. Used by Object.prototype.toString().
Symbol.unscopables: An object value of whose own and inherited property names are excluded from the with environment bindings of the associated object.

Static methods

Symbol.for(key): Searches for existing symbols with the given key and returns it if found. Otherwise a new symbol gets created in the global symbol registry with key.
Symbol.keyFor(sym): Retrieves a shared symbol key from the global symbol registry for the given symbol.

Instance properties

Symbol.prototype.description: A read-only string containing the description of the symbol.

Instance methods

Symbol.prototype.toSource(): Returns a string containing the source of the Symbol object. Overrides the Object.prototype.toSource() method.
Symbol.prototype.toString(): Returns a string containing the description of the Symbol. Overrides the Object.prototype.toString() method.
Symbol.prototype.valueOf(): Returns the primitive value of the Symbol object. Overrides the Object.prototype.valueOf() method.
Symbol.prototype[@@toPrimitive]: Returns the primitive value of the Symbol object.

Examples

Using the typeof operator with symbols

The typeof operator can help you to identify symbols.

typeof Symbol() === 'symbol'
typeof Symbol('foo') === 'symbol'
typeof Symbol.iterator === 'symbol'

Symbol type conversions

Some things to note when working with type conversion of symbols.

When trying to convert a symbol to a number, a TypeError will be thrown (e.g. +sym or sym | 0).
When using loose equality, Object(sym) == sym returns true.
Symbol("foo") + "bar" throws a TypeError (can't convert symbol to string). This prevents you from silently creating a new string property name from a symbol, for example.
The "safer" String(sym) conversion works like a call to Symbol.prototype.toString() with symbols, but note that new String(sym) will throw.

Symbols and for...in iteration

Symbols are not enumerable in for...in iterations. In addition, Object.getOwnPropertyNames() will not return symbol object properties, however, you can use Object.getOwnPropertySymbols() to get these.

let obj = {}

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

for (let i in obj) {
   console.log(i)  // logs "c" and "d"
}

Symbols and JSON.stringify()

Symbol-keyed properties will be completely ignored when using JSON.stringify():

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

For more details, see JSON.stringify().

Symbol wrapper objects as property keys

When a Symbol wrapper object is used as a property key, this object will be coerced to its wrapped symbol:

let sym = Symbol('foo')
let obj = {[sym]: 1}
obj[sym]             // 1
obj[Object(sym)]     // still 1

Specifications

Specification

ECMAScript (ECMA-262)The definition of 'Symbol' in that specification.

Browser compatibility

Update compatibility data on GitHub

	Desktop						Mobile						Server
	Chrome	Edge	Firefox	Internet Explorer	Opera	Safari	Android webview	Chrome for Android	Firefox for Android	Opera for Android	Safari on iOS	Samsung Internet	Node.js
`Symbol`	Chrome Full support 38	Edge Full support 12 Notes' Full support 12 Notes' Notes' Edge 12 included Symbol properties in `JSON.stringify()` output.	Firefox Full support 36	IE No support No	Opera Full support 25	Safari Full support 9	WebView Android Full support 38	Chrome Android Full support 38	Firefox Android Full support 36	Opera Android Full support 25	Safari iOS Full support 9	Samsung Internet Android Full support 3.0	nodejs Full support 0.12
`Symbol()` constructor	Chrome Full support 38	Edge Full support 12	Firefox Full support 36	IE No support No	Opera Full support 25	Safari Full support 9	WebView Android Full support 38	Chrome Android Full support 38	Firefox Android Full support 36	Opera Android Full support 25	Safari iOS Full support 9	Samsung Internet Android Full support 3.0	nodejs Full support 0.12
`asyncIterator`	Chrome Full support 63	Edge Full support 79	Firefox Full support 57	IE No support No	Opera Full support 50	Safari Full support 11.1	WebView Android Full support 63	Chrome Android Full support 63	Firefox Android No support No	Opera Android Full support 46	Safari iOS No support No	Samsung Internet Android Full support 8.0	nodejs Full support 10.0.0
`description`	Chrome Full support 70	Edge Full support 79	Firefox Full support 63	IE No support No	Opera Full support 57	Safari Full support 12.1 Full support 12.1 Partial support 12 Notes' Notes' No support for an undefined description.	WebView Android Full support 70	Chrome Android Full support 70	Firefox Android Full support 63	Opera Android Full support 49	Safari iOS Full support 12.2 Full support 12.2 Partial support 12 Notes' Notes' No support for an undefined description.	Samsung Internet Android Full support 10.0	nodejs Full support 11.0.0
`for`	Chrome Full support 40	Edge Full support 12	Firefox Full support 36	IE No support No	Opera Full support 27	Safari Full support 9	WebView Android Full support 40	Chrome Android Full support 40	Firefox Android Full support 36	Opera Android Full support 27	Safari iOS Full support 9	Samsung Internet Android Full support 4.0	nodejs Full support 0.12
`hasInstance`	Chrome Full support 50	Edge Full support 15	Firefox Full support 50	IE No support No	Opera Full support 37	Safari Full support 10	WebView Android Full support 50	Chrome Android Full support 50	Firefox Android Full support 50	Opera Android Full support 37	Safari iOS Full support 10	Samsung Internet Android Full support 5.0	nodejs Full support 6.5.0 Full support 6.5.0 Full support 6.0.0 Disabled' Disabled' From version 6.0.0: this feature is behind the `--harmony` runtime flag.
`isConcatSpreadable`	Chrome Full support 48	Edge Full support 15	Firefox Full support 48	IE No support No	Opera Full support 35	Safari Full support 10	WebView Android Full support 48	Chrome Android Full support 48	Firefox Android Full support 48	Opera Android Full support 35	Safari iOS Full support 10	Samsung Internet Android Full support 5.0	nodejs Full support 6.0.0
`iterator`	Chrome Full support 43	Edge Full support 12	Firefox Full support 36	IE No support No	Opera Full support 30	Safari Full support 10	WebView Android Full support 43	Chrome Android Full support 43	Firefox Android Full support 36	Opera Android Full support 30	Safari iOS Full support 10	Samsung Internet Android Full support 4.0	nodejs Full support 0.12
`keyFor`	Chrome Full support 40	Edge Full support 12	Firefox Full support 36	IE No support No	Opera Full support 27	Safari Full support 9	WebView Android Full support 40	Chrome Android Full support 40	Firefox Android Full support 36	Opera Android Full support 27	Safari iOS Full support 9	Samsung Internet Android Full support 4.0	nodejs Full support 0.12
`match`	Chrome Full support 50	Edge Full support 79	Firefox Full support 40	IE No support No	Opera Full support 37	Safari Full support 10	WebView Android Full support 50	Chrome Android Full support 50	Firefox Android Full support 40	Opera Android Full support 37	Safari iOS Full support 10	Samsung Internet Android Full support 5.0	nodejs Full support 6.0.0
`matchAll`	Chrome Full support 73	Edge Full support 79	Firefox Full support 67	IE No support No	Opera Full support 60	Safari Full support 13	WebView Android Full support 73	Chrome Android Full support 73	Firefox Android Full support 67	Opera Android Full support 52	Safari iOS Full support 13	Samsung Internet Android No support No	nodejs Full support 12.0.0
`replace`	Chrome Full support 50	Edge Full support 79	Firefox Full support 49	IE No support No	Opera Full support 37	Safari Full support 10	WebView Android Full support 50	Chrome Android Full support 50	Firefox Android Full support 49	Opera Android Full support 37	Safari iOS Full support 10	Samsung Internet Android Full support 5.0	nodejs Full support 6.0.0
`search`	Chrome Full support 50	Edge Full support 79	Firefox Full support 49	IE No support No	Opera Full support 37	Safari Full support 10	WebView Android Full support 50	Chrome Android Full support 50	Firefox Android Full support 49	Opera Android Full support 37	Safari iOS Full support 10	Samsung Internet Android Full support 5.0	nodejs Full support 6.0.0
`species`	Chrome Full support 51	Edge Full support 13	Firefox Full support 41	IE No support No	Opera Full support 38	Safari Full support 10	WebView Android Full support 51	Chrome Android Full support 51	Firefox Android Full support 41	Opera Android Full support 41	Safari iOS Full support 10	Samsung Internet Android Full support 5.0	nodejs Full support 6.5.0 Full support 6.5.0 Full support 6.0.0 Disabled' Disabled' From version 6.0.0: this feature is behind the `--harmony` runtime flag.
`split`	Chrome Full support 50	Edge Full support 79	Firefox Full support 49	IE No support No	Opera Full support 37	Safari Full support 10	WebView Android Full support 50	Chrome Android Full support 50	Firefox Android Full support 49	Opera Android Full support 37	Safari iOS Full support 10	Samsung Internet Android Full support 5.0	nodejs Full support 6.0.0
`toPrimitive`	Chrome Full support 47	Edge Full support 15	Firefox Full support 44	IE No support No	Opera Full support 34	Safari Full support 10	WebView Android Full support 47	Chrome Android Full support 47	Firefox Android Full support 44	Opera Android Full support 34	Safari iOS Full support 10	Samsung Internet Android Full support 5.0	nodejs Full support 6.0.0
`toSource` Non-standard'	Chrome No support No	Edge No support No	Firefox No support 36 — 74 Notes' No support 36 — 74 Notes' Notes' Starting in Firefox 74, `toSource()` is no longer available for use by web content. It is still allowed for internal and privileged code.	IE No support No	Opera No support No	Safari No support No	WebView Android No support No	Chrome Android No support No	Firefox Android Full support 36	Opera Android No support No	Safari iOS No support No	Samsung Internet Android No support No	nodejs No support No
`toString`	Chrome Full support 38	Edge Full support 12	Firefox Full support 36	IE No support No	Opera Full support 25	Safari Full support 9	WebView Android Full support 38	Chrome Android Full support 38	Firefox Android Full support 36	Opera Android Full support 25	Safari iOS Full support 9	Samsung Internet Android Full support 3.0	nodejs Full support 0.12
`toStringTag`	Chrome Full support 49	Edge Full support 15	Firefox Full support 51	IE No support No	Opera Full support 36	Safari Full support 10	WebView Android Full support 49	Chrome Android Full support 49	Firefox Android Full support 51	Opera Android Full support 36	Safari iOS Full support 10	Samsung Internet Android Full support 5.0	nodejs Full support 6.0.0 Full support 6.0.0 Full support 4.0.0 Disabled' Disabled' From version 4.0.0: this feature is behind the `--harmony` runtime flag.
`unscopables`	Chrome Full support 45	Edge Full support 12	Firefox Full support 48	IE No support No	Opera Full support 32	Safari Full support 9	WebView Android Full support 45	Chrome Android Full support 45	Firefox Android Full support 48	Opera Android Full support 32	Safari iOS Full support 9	Samsung Internet Android Full support 5.0	nodejs Full support 0.12
`valueOf`	Chrome Full support 38	Edge Full support 12	Firefox Full support 36	IE No support No	Opera Full support 25	Safari Full support 9	WebView Android Full support 38	Chrome Android Full support 38	Firefox Android Full support 36	Opera Android Full support 25	Safari iOS Full support 9	Samsung Internet Android Full support 3.0	nodejs Full support 0.12
`@@toPrimitive`	Chrome Full support 47	Edge Full support 15	Firefox Full support 44	IE No support No	Opera Full support 34	Safari Full support 10	WebView Android Full support 47	Chrome Android Full support 47	Firefox Android Full support 44	Opera Android Full support 34	Safari iOS Full support 10	Samsung Internet Android Full support 5.0	nodejs Full support 6.0.0

Legend

Full support: Full support
No support: No support
Non-standard. Expect poor cross-browser support.': Non-standard. Expect poor cross-browser support.
See implementation notes.': See implementation notes.
User must explicitly enable this feature.': User must explicitly enable this feature.