TypedArray[Symbol.species]

The value of the constructor (this) on which get [Symbol.species] was called. The return value is used to construct return values from typed array methods that create new typed arrays.

The [Symbol.species] accessor property returns the default constructor for typed array objects. Subclass constructors may override it to change the constructor assignment. The default implementation is basically:

// Hypothetical underlying implementation for illustration
class TypedArray {
  static get [Symbol.species]() {
    return this;
  }
}

Because of this polymorphic implementation, [Symbol.species] of derived subclasses would also return the constructor itself by default.

class SubTypedArray extends Int8Array {}
SubTypedArray[Symbol.species] === SubTypedArray; // true

When calling typed array methods that do not mutate the existing array but return a new array instance (for example, filter() and map()), the array's constructor[Symbol.species] will be accessed. The returned constructor will be used to construct the return value of the typed array method.

However, unlike Array[Symbol.species], when using [Symbol.species] to create new typed arrays, the language will make sure that the newly created array is a proper typed array and has the same content type as the original array — for example, you can't create a BigInt64Array from a Float64Array, or create a non-BigInt array from a BigInt array. Doing so throws a TypeError.

class BadArray extends Int8Array {
  static get [Symbol.species]() {
    return Array;
  }
}
new BadArray(1).map(() => 0); // TypeError: Method %TypedArray%.prototype.map called on incompatible receiver [object Array]

class BadArray2 extends Int8Array {
  static get [Symbol.species]() {
    return BigInt64Array;
  }
}
new BadArray2(1).map(() => 0n); // TypeError: TypedArray.prototype.map constructed typed array of different content type from |this|

The [Symbol.species] property returns the default constructor function, which is one of the typed array constructors itself for any given typed array constructor.

Int8Array[Symbol.species]; // function Int8Array()
Uint8Array[Symbol.species]; // function Uint8Array()
Float32Array[Symbol.species]; // function Float32Array()

In an instance of a custom TypedArray subclass, such as MyTypedArray, the MyTypedArray species is the MyTypedArray constructor. However, you might want to overwrite this, in order to return a parent typed array object in your derived class methods:

class MyTypedArray extends Uint8Array {
  // Overwrite MyTypedArray species to the parent Uint8Array constructor
  static get [Symbol.species]() {
    return Uint8Array;
  }
}

• TypedArray
• Symbol.species