Why ArraySpeciesCreate is called in Array.prototype.splice?

Question

ES6 mentioned ArraySpeciesCreate is called in Array.prototype.splice in step 12, and the later creates a new array according to is original array. Why ArraySpeciesCreate is required to be called to create a copy of the original array? Does Array.prototype.splice operate in the original array directly like below?

var a = [1, 2, 3];
a.splice(0, 1); // a is [2, 3] after this statement

Answer 1

Why ArraySpeciesCreate is required to be called to create a copy of the original array?

It doesn't create a copy of the original array (more below).

Does Array.prototype.splice operate in the original array directly...?

Yes.

splice does two things:

1. It modifies the array you call it on, and

2. It creates a new array that contains any entries you remove

ArraySpeciesCreate is used because of #2, so that with a subclassed array, you get back an instance of the subclass:

 class CustomArray extends Array { } let main = new CustomArray("a", "b", "c", "d"); let deleted = main.splice(1, 2); // Remove and return b and c console.log("main", main); // ["a", "d"] console.log("deleted", deleted); // ["b", "c"] console.log(deleted instanceof CustomArray); // true 

Note how the array we got back from splice is an instance of CustomArray . That's because of the use of ArraySpeciesCreate that you flagged up in your question. From the description of ArraySpeciesCreate in the spec:

The abstract operation ArraySpeciesCreate with arguments originalArray and length is used to specify the creation of a new Array object using a constructor function that is derived from originalArray .

Note that bit at the end. Basically, ArraySpeciesCreate uses the original array's constructor property to build the new array.