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Symbol.species

The Symbol.species static data property represents the well-known symbol @@species. Methods that create copies of an object may look up this symbol on the object for the constructor function to use when creating the copy.

Warning: The existence of @@species allows execution of arbitrary code and may create security vulnerabilities. It also makes certain optimizations much harder. Engine implementers are investigating whether to remove this feature. Avoid relying on it if possible.

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Value

The well-known symbol @@species.

Property attributes of Symbol.species
Writable no
Enumerable no
Configurable no

Description

The @@species accessor property allows subclasses to override the default constructor for objects. This specifies a protocol about how instances should be copied. For example, when you use copying methods of arrays, such as map(). the map() method uses instance.constructor[Symbol.species] to get the constructor for constructing the new array. For more information, see subclassing built-ins.

All built-in implementations of @@species return the this value, which is the current instance's constructor. This allows copying methods to create instances of derived classes rather than the base class — for example, map() will return an array of the same type as the original array.

Examples

Using species

You might want to return Array objects in your derived array class MyArray. For example, when using methods such as map() that return the default constructor, you want these methods to return a parent Array object, instead of the MyArray object. The species symbol lets you do this:

js
class MyArray extends Array {
  // Overwrite species to the parent Array constructor
  static get [Symbol.species]() {
    return Array;
  }
}
const a = new MyArray(1, 2, 3);
const mapped = a.map((x) => x * x);

console.log(mapped instanceof MyArray); // false
console.log(mapped instanceof Array); // true

Specifications

Browser compatibility

Desktop Mobile Server
Chrome Edge Firefox Opera Safari Chrome Android Firefox for Android Opera Android Safari on IOS Samsung Internet WebView Android Deno Node.js
species 51 13 41 38 10 51 41 41 10 5.0 51 1.0 6.5.0

See also

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Licensed under the Creative Commons Attribution-ShareAlike License v2.5 or later.
https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Symbol/species