Move an array element from one array position to another

arrays, javascript

Solution

If you'd like a version on npm, array-move is the closest to this answer, although it's not the same implementation. See its usage section for more details. The previous version of this answer (that modified Array.prototype.move) can be found on npm at array.prototype.move.

I had fairly good success with this function:

function array_move(arr, old_index, new_index) {
    if (new_index >= arr.length) {
        var k = new_index - arr.length + 1;
        while (k--) {
            arr.push(undefined);
        }
    }
    arr.splice(new_index, 0, arr.splice(old_index, 1)[0]);
    return arr; // for testing
};

// returns [2, 1, 3]
console.log(array_move([1, 2, 3], 0, 1));

Note that the last `return` is simply for testing purposes: `splice` performs operations on the array in-place, so a return is not necessary. By extension, this `move` is an in-place operation. If you want to avoid that and return a copy, use `slice`.

Stepping through the code:

- If `new_index` is greater than the length of the array, we want (I presume) to pad the array properly with new `undefined`s. This little snippet handles this by pushing `undefined` on the array until we have the proper length.

- Then, in `arr.splice(old_index, 1)[0]`, we splice out the old element. `splice` returns the element that was spliced out, but it's in an array. In our above example, this was `[1]`. So we take the first index of that array to get the raw `1` there.

- Then we use `splice` to insert this element in the new_index's place. Since we padded the array above if `new_index > arr.length`, it will probably appear in the right place, unless they've done something strange like pass in a negative number.

A fancier version to account for negative indices:

function array_move(arr, old_index, new_index) {
    while (old_index < 0) {
        old_index += arr.length;
    }
    while (new_index < 0) {
        new_index += arr.length;
    }
    if (new_index >= arr.length) {
        var k = new_index - arr.length + 1;
        while (k--) {
            arr.push(undefined);
        }
    }
    arr.splice(new_index, 0, arr.splice(old_index, 1)[0]);
    return arr; // for testing purposes
};
    
// returns [1, 3, 2]
console.log(array_move([1, 2, 3], -1, -2));

Which should account for things like `array_move([1, 2, 3], -1, -2)` properly (move the last element to the second to last place). Result for that should be `[1, 3, 2]`.

Either way, in your original question, you would do `array_move(arr, 0, 2)` for `a` after `c`. For `d` before `b`, you would do `array_move(arr, 3, 1)`.

Problem

I'm having a hard time figuring out how to move an element of an array. For example, given the following: ``` var array = [ 'a', 'b', 'c', 'd', 'e']; ``` How can I write a function to move the element `'d'` to the left of `'b'` ? Or `'a'` to the right of `'c'`? After moving the elements, the indexes of the rest of the elements should be updated. The resulting array would be: ``` array = ['a', 'd', 'b', 'c', 'e'] ``` This seems like it should be pretty simple, but I can't wrap my head around it.

Original source

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