JavaScript array.prototype how to set 'this' to a new array obtained by a method

Question

I've attempted to define a randomize method to Array.prototype like so :

Array.prototype.randomize = function() { // Yes, this method does cause the array to have 'holes', but its good enough for this example/demonstration
    var r = new Array(this.length); // Possible but somewhat unwanted to randomize the array in its place, therefore it has to be set to this new array later on.

    this.forEach(function(e) {
        r[Math.floor(Math.random() * r.length)] = e;
    });

    // how do I set 'this' to the variable r? (In order to change the array to the new and randomized array 'r')
    return r;

}

This method does return the randomized array, but how do I change the array itself as well?

What if Math.floor(Math.random() * r.length) is the same for each e? — joews
– joews, Commented Oct 20, 2016 at 22:24
You need to mutate the original array. Learn about in-place shuffling algorithms. — SLaks
– SLaks, Commented Oct 20, 2016 at 22:24
something else - your function doesn't shuffle the array anyway, you could end up with many "holes" in r — Jaromanda X
– Jaromanda X, Commented Oct 20, 2016 at 22:24
@WEB_UI You can change the values inside of the current array. this[i] = x. — Mike Cluck
– Mike Cluck, Commented Oct 20, 2016 at 22:25

joews · Accepted Answer · 2016-10-20 22:37:23Z

3

As the comments say, changing an array in place in place is a better way to shuffle.

But if you did need to replace all of the elements in one go, you could use Array#splice:

Array.prototype.randomize = function() { 
    var r = /* the algorithm to get a replacement array... */;

    this.splice(0, r.length, ...r);
    return this;
}

... is the spread operator. It's part of ES2015.

Spread operator compatibility table

edited Oct 20, 2016 at 22:37

answered Oct 20, 2016 at 22:33

joews

30.4k11 gold badges82 silver badges91 bronze badges

Sign up to request clarification or add additional context in comments.

1 Comment

Mystical Over a year ago

Great! Problem solved (although I am looking forward to shuffle the array in its place later on).

Community · Accepted Answer · 2017-05-23 12:33:23Z

3

not possible to randomize the array in its place

Wrong.

how do I set 'this' to the variable array 'r' in order to change the array to the new one?

That's impossible in JavaScript. You cannot overwrite an object, not via the this reference and not via a normal variable, you have to actually mutate its properties. If you want to overwrite the variable in which the array reference is stored, you need to explicitly assign to that variable; you cannot do it via a method (since JS does not allow pass-by-reference) and you can only overwrite this reference, not all variables that might contain it.

edited May 23, 2017 at 12:33

CommunityBot

11 silver badge

answered Oct 20, 2016 at 23:07

Bergi

671k162 gold badges1k silver badges1.5k bronze badges

1 Comment

Mystical Over a year ago

Possible but somewhat unwanted to randomize the array in its place, and that's because I just wanted to know how it is possible to change the this reference which I now realize is impossible because apparently "JS does not allow pass-by-reference." Which is of course, totally fine. Good point though!

Collectives™ on Stack Overflow

JavaScript array.prototype how to set 'this' to a new array obtained by a method

2 Answers 2

1 Comment

1 Comment

Your Answer

Linked

Hot Network Questions

Collectives™ on Stack Overflow

2 Answers 2

1 Comment

1 Comment

Your Answer

Sign up or log in

Post as a guest

Linked

Related