javascript syntax question about splice in this statement -> copy.push(array.splice(i,1)[0])

Question

So this is a syntax question, I have been reading the MDN docs but i can't find a similar example. I got this particular code snippet from here: https://bost.ocks.org/mike/shuffle/ as I was reading on randomizing arrays.

Here is the full function:

    function shuffle(array) {
        var copy = [], n = array.length, i;

        // While there remain elements to shuffle…
        while (n) {

            // Pick a remaining element…
            i = Math.floor(Math.random() * n--);

            // And move it to the new array.
            copy.push(array.splice(i, 1)[0]);
        }

        return copy;
    }

Now, I have used it and modified it to fit my needs and it worked fine (although the splicing was a nightmare because it kept destroying my original data) but the point here is this line:

copy.push(array.splice(i, 1)[0]);

I hate not understanding something that would seem basic, but what is the [0] doing in this case? If i take it out the whole thing breaks, but im not sure if it;s part of the splice, or the push, or if its an index of the new array or the spliced array. IF anyone can help shed some light on this i would be very grateful!

Answer 1

Let's break down

copy.push(array.splice(i, 1)[0]);

in the order that it is "executed"

array.splice(i, 1);

array.splice(n, m) removes m elements starting and index n and returns these removed elements as an array ... so in this case you get an array of length 1 - let's call it x

substituting x for array.splice(i, 1) in the original code, we now have

copy.push(x[0]);

x[0] is the one and only element removed from array - which is pushed on to copy array

to prevent the incoming array from being mutated

function shuffle(array) {
    var copy = [], n = array.length, i;
    array = array.slice();
    // rest of code
}

---

By the way, you could also have done

copy.push(array.splice(i, 1).pop());

or

copy.push(array.splice(i, 1).shift());