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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:

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

Specification
ECMAScript Language Specification # sec-symbol.species

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