Symbol

A symbol is a unique and immutable data type. It may be used as an identifier for object properties. The Symbol object is an implicit object wrapper for the symbol primitive data type.

Syntax

Symbol([description])

Parameters

description Optional

Optional, string. A description of the symbol which can be used for debugging but not to access the symbol itself.

Description

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

var sym1 = Symbol();
var sym2 = Symbol("foo");
var 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:

var 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:

var sym = Symbol("foo");
typeof sym;     // "symbol" 
var 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.

Properties

Symbol.length

Length property whose value is 0.

Symbol.prototype

Represents the prototype for the Symbol constructor.

Well-known symbols

In addition to your own symbols, JavaScript has some built-in symbols which represent internal language behaviors which were not exposed to developers in ECMAScript 5 and before. These symbols can be accessed using the following properties:

Iteration symbols

Symbol.iterator

A method returning the default iterator for an object. Used by for...of.

Regular expression symbols

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.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().

Other symbols

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.unscopables

An object value of whose own and inherited property names are excluded from the with environment bindings of the associated object.

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().

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 this key.

Symbol.keyFor(sym)

Retrieves a shared symbol key from the global symbol registry for the given symbol.

Symbol prototype

All Symbols inherit from Symbol.prototype.

Properties

Symbol.prototype.constructor

Returns the function that created an instance's prototype. This is the Symbol function by default.

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 of 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.

var obj = {};

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

for (var 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:

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

Specifications

Browser compatibility

See also

• Glossary: Symbol data type
• typeof
• Data types and data structures
• "ES6 In Depth: Symbols" on hacks.mozilla.org