Symbol
Symbol
is a built-in object whose constructor returns a symbol
primitive — also called a Symbol value or just a Symbol — that's guaranteed to be unique. Symbols are often used to add unique property keys to an object that won't collide with keys any other code might add to the object, and which are hidden from any mechanisms other code will typically use to access the object. That enables a form of weak encapsulation, or a weak form of information hiding.
Every Symbol()
call is guaranteed to return a unique Symbol. Every Symbol.for("key")
call will always return the same Symbol for a given value of "key"
. When Symbol.for("key")
is called, if a Symbol with the given key can be found in the global Symbol registry, that Symbol is returned. Otherwise, a new Symbol is created, added to the global Symbol registry under the given key, and returned.
Description
To create a new primitive Symbol, you write Symbol()
with an optional string as its description:
const sym1 = Symbol();
const sym2 = Symbol("foo");
const 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
:
const 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:
const sym = Symbol("foo");
typeof sym; // "symbol"
const symObj = Object(sym);
typeof symObj; // "object"
Because symbols are the only primitive data type that has reference identity (that is, you cannot create the same symbol twice), they behave like objects in some way. For example, they are garbage collectable and can therefore be stored in WeakMap
, WeakSet
, WeakRef
, and FinalizationRegistry
objects.
Shared Symbols in the global Symbol registry
The above syntax using the Symbol()
function will create a Symbol whose value remains unique throughout the lifetime of the program. 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.
Note that the "global Symbol registry" is only a fictitious concept and may not correspond to any internal data structure in the JavaScript engine — and even if such a registry exists, its content is not available to the JavaScript code, except through the for()
and keyFor()
methods.
The method Symbol.for(tokenString)
takes a string key and returns a symbol value from the registry, while Symbol.keyFor(symbolValue)
takes a symbol value and returns the string key corresponding to it. Each is the other's inverse, so the following is true
:
Symbol.keyFor(Symbol.for("tokenString")) === "tokenString"; // true
Because registered symbols can be arbitrarily created anywhere, they behave almost exactly like the strings they wrap. Therefore, they are not guaranteed to be unique and are not garbage collectable. Therefore, registered symbols are disallowed in WeakMap
, WeakSet
, WeakRef
, and FinalizationRegistry
objects.
Well-known Symbols
All static properties of the Symbol
constructor are Symbols themselves, whose values are constant across realms. They are known as well-known Symbols, and their purpose is to serve as "protocols" for certain built-in JavaScript operations, allowing users to customize the language's behavior. For example, if a constructor function has a method with Symbol.hasInstance
as its name, this method will encode its behavior with the instanceof
operator.
Prior to well-known Symbols, JavaScript used normal properties to implement certain built-in operations. For example, the JSON.stringify
function will attempt to call each object's toJSON()
method, and the String
function will call the object's toString()
and valueOf()
methods. However, as more operations are added to the language, designating each operation a "magic property" can break backward compatibility and make the language's behavior harder to reason with. Well-known Symbols allow the customizations to be "invisible" from normal code, which typically only read string properties.
Note: The spec used to use the notation @@<symbol-name>
to denote well-known symbols. For example, Symbol.hasInstance
was written as @@hasInstance
, and the Array.prototype[Symbol.iterator]()
method would be called Array.prototype[@@iterator]()
. This notation is no longer used in the spec, but you may still see it in older documentation or discussions.
Well-known symbols do not have the concept of garbage collectability, because they come in a fixed set and are unique throughout the lifetime of the program, similar to intrinsic objects such as Array.prototype
, so they are also allowed in WeakMap
, WeakSet
, WeakRef
, and FinalizationRegistry
objects.
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()
-
Returns primitive values of type Symbol. Throws an error when called with
new
.
Static properties
The static properties are all well-known Symbols. In these Symbols' descriptions, we will use language like "Symbol.hasInstance
is a method determining…", but bear in mind that this is referring to the semantic of an object's method having this Symbol as the method name (because well-known Symbols act as "protocols"), not describing the value of the Symbol itself.
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.species
-
A constructor function that is used to create derived objects.
Symbol.split
-
A method that splits a string at the indices that match a regular expression. Used by
String.prototype.split()
. 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()
-
Searches for existing registered Symbols in the global Symbol registry with the given
key
and returns it if found. Otherwise a new Symbol gets created and registered withkey
. Symbol.keyFor()
-
Retrieves a shared Symbol key from the global Symbol registry for the given Symbol.
Instance properties
These properties are defined on Symbol.prototype
and shared by all Symbol
instances.
Symbol.prototype.constructor
-
The constructor function that created the instance object. For
Symbol
instances, the initial value is theSymbol
constructor. Symbol.prototype.description
-
A read-only string containing the description of the Symbol.
Symbol.prototype[Symbol.toStringTag]
-
The initial value of the
[Symbol.toStringTag]
property is the string"Symbol"
. This property is used inObject.prototype.toString()
. However, becauseSymbol
also has its owntoString()
method, this property is not used unless you callObject.prototype.toString.call()
with a symbol asthisArg
.
Instance methods
Symbol.prototype.toString()
-
Returns a string containing the description of the Symbol. Overrides the
Object.prototype.toString()
method. Symbol.prototype.valueOf()
-
Returns the Symbol. Overrides the
Object.prototype.valueOf()
method. Symbol.prototype[Symbol.toPrimitive]()
-
Returns the Symbol.
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
orsym | 0
). - When using loose equality,
Object(sym) == sym
returnstrue
. Symbol("foo") + "bar"
throws aTypeError
(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 toSymbol.prototype.toString()
with Symbols, but note thatnew 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.
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"
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:
const sym = Symbol("foo");
const obj = { [sym]: 1 };
obj[sym]; // 1
obj[Object(sym)]; // still 1
Specifications
Specification |
---|
ECMAScript Language Specification # sec-symbol-objects |
Browser compatibility
BCD tables only load in the browser
See also
- Polyfill of
Symbol
incore-js
typeof
- JavaScript data types and data structures
- ES6 In Depth: Symbols on hacks.mozilla.org (2015)