这个Array.apply方法如何工作？

Question

I understand you can pass an array to Function.prototype.apply but recently I've come across this code (it was written as a good way to create an array with undefined values as opposed to empty slots or holes);

var a = Array.apply( null, { length: 3 } );

I can't understand how this code works starting with the second argument. Is there another syntax that could be used to make it more understandable what's going on? What's going on when we pass an actual object to apply as opposed to an array? Can we somehow translate this object to an array to get this result? I've tried many things to do this, but without success.

Answer 1

Oh, that is just horrible.

So, when you call Array as a function, it doesn't really care what this is so you can tell it that this is null .

The second argument for apply is an array-like object which gives the list of arguments.

If you had [0, 0, 0] then that would be like this object:

{
    0: 0,
    1: 0,
    2: 0,
    length: 3
}

… because arrays get a length property which equals the value of their highest index. A real array would get other properties from the prototype, but we (and apply , and Array ) don't care about them.

The arguments you pass to Array become its initial values.

Now, it seems that (internally) Array checks the length of its arguments and then sets its internal values using something like:

for (var i = 0; i < arguments.length; i++) {
    internal_array_values.push(arguments[0])
}

And if you have an object which consists solely of { length: 3 } it is going to get undefined as the value of each of those three arguments giving you [undefined, undefined, undefined] .

Answer 2

http://www.2ality.com/2012/07/apply-tricks.html

With apply and Array (which can be used as either a function or a constructor), you can turn holes into undefined elements:

 Array.apply(null, ["a",,"b"])
  // [ 'a', undefined, 'b' ]