Symbol.species
Baseline Widely available
This feature is well established and works across many devices and browser versions. It’s been available across browsers since September 2015.
The Symbol.species
static data property represents the well-known symbol 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 [Symbol.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 Symbol.species
.
Property attributes of Symbol.species | |
---|---|
Writable | no |
Enumerable | no |
Configurable | no |
Description
The [Symbol.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 [Symbol.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
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