Feature Request: _.move() element in an array

[blindstuff]

I needed this in the implementation of a drag and drop gallery and found it was not on the lodash docs. If this already exists or is implemented in a different way I apologize as I could not find it.

A simple method to move an element inside of an array from one position to another.

_.move = function(array,fromIndex,toIndex){
    array.splice(toIndex, 0, array.splice(fromIndex, 1)[0] );
}

Example usage

_.move([1,2,3,4,5],0,2);
// -> [2,3,1,4,5]

Solution taken from (and more detail / verifications needed available at):
http://stackoverflow.com/a/5306832/474883

[jdalton]

Thanks @blindstuff!

This looks like a nice complement to the _.insert proposal. While working with splice is a bit cumbersome I'll defer to it for now. We'll keep an eye on the popularity of the request.

[granteagon]

I created a repo with simple tests here and published lodash-move on npm. Hope it helps.

[jtheoof]

As noted by @granteagon, this would be a nice touch for drag and drops :)

[c-dante]

Would it make more sense to just add a permutation function? Reorders arrays based on a permutation function or re-index array? Maybe a list of swaps to perform or what have you?

const arr = [1, 2, 3, 4, 5]
_.permute(arr, [0, 2, 1, 3, 4]); // [1, 3, 2, 4, 5]

const fn = (idx) => idx + 1;
_.permute(arr, fn); // [_, 1, 2, 3, 4, 5] ?

No matter what, questions are:

• What do you expect to happen if you move out of bounds?
• What happens if you move multiple elements to the same index?
• Can you only do 1 swap at a time?
• Does it make sense to just be _.move(arr, fromIdx, toIdx) or something more... useful?

[granteagon]

@c-dante I think undefined stuff should either raise an error or just insert undefined values. I like the permute idea, but a definite flaw I see is the array move is hand coded. If you were to use it in real code, wouldn't you still need to .slice() or something to get the array of numbers illustrated above?

[c-dante]

You can implement it in a few ways -- especially since you're creating a new array each time.

Your implementation uses spread a bunch to reorder it, you could use the permute array to select elements, I'd avoid splice since it mutates the input (although that might be desired in some cases).

Yes, my example is hard coded, but there's no reason you the signature has to be that. Second example is a mapping function that goes from old to new index. Alternatively, you can have a vararg of swaps.

_.permute(arr, newIndices);
_.permute(arr, ...swaps);
_.permute(arr, mapFn);

are a few signiture ideas.

[jtheoof]

I like the _.permute idea as well as long as it remains easy for "simple swaps" (drag and drops). But it looks like _.permute(arr, ...swaps) has this covered.

[granteagon]

Interesting. That would still work for the drag n' drop examples above so I'm good. permute is a better name than arrayMove IMO.

[granteagon]

@c-dante et. al, I created a _.permute mixin on github and npm you can find here: https://github.com/granteagon/lodash-permute

Feedback welcome!