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.

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"

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.

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.

Symbol()

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

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.

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.

Symbol.prototype.description

A read-only string containing the description of the Symbol.

Symbol.prototype.toSource()

Returns a string containing the source of the Symbol. 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 Symbol. Overrides the Object.prototype.valueOf() method.

Symbol.prototype[@@toPrimitive]

Returns the Symbol.

The typeof operator can help you to identify Symbols.

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

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 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"
}

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

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

For more details, see JSON.stringify().

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

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