Move every other value from array into a new array

Question

I have two one-dimensional arrays, a and b. a has values and b is empty. The length of a is an even number. I'd like to remove every other value from a and move them to b, in the same order as they were placed in a.

var a = [1, 2, 3, 4, 5, 6], b = [];

becomes

var a = [1, 3, 5], b = [2, 4, 6];

I figured that filter would do the trick but I'm not that happy with the performance of it since the average length of a is 300-400.

b = a.filter((i, idx) => {
    return idx % 2 == 0;
});
a = a.filter((i, idx) => {
    return idx % 2 == 1;
});

I've also been looking at lodash to see if that library had anything that might help me and the only function that's near what I'm looking for is _.chunk(array, \[size=1\]).

I appreciate any and all help to help me figure out a better, faster way to do this.

Answer 1

Since you mentioned lodash you could do this with _.partition:

let a = [1, 2, 3, 4, 5, 6];
let b = [];
let i = -1;

[a, b] = _.partition(a, (item) => i++ % 2);

console.log(a);
console.log(b);

<script src="https://cdn.jsdelivr.net/lodash/4.17.4/lodash.min.js"></script>

Partition's predicate is the identity function, which doesn't include the index of the item, so this comes with a compromise of an external index i.

Of course, you could always wrap this functionality into it's own function:

const splitEvenOdd = (array, i = -1) => _.partition(array, (item) => i++ % 2);

let a = [1, 2, 3, 4, 5, 6];
let b = [];

[a, b] = splitEvenOdd(a);

console.log(a);
console.log(b);

<script src="https://cdn.jsdelivr.net/lodash/4.17.4/lodash.min.js"></script>

Answer 2

Vanilla JS ES5, simple and clean.

var a = [1, 2, 3, 4, 5, 6], b = [];

for(var i = a.length-1; i >= 0; i--) {
  if(i % 2 === 1) {
    b.unshift(a.splice(i, 1)[0])
  }
}

Basically, it is iterating through a backwards, and if the condition is true splicing the item und adding it as first item of b.