# Batch copy to array (seeking champion)

**URL:** <https://es.discourse.group/t/batch-copy-to-array-seeking-champion/362>\
**Category:** 💡 Ideas\
**Tags:** proposal\
**Created:** [June 16, 2020, 5:40am UTC](https://es.discourse.group/t/batch-copy-to-array-seeking-champion/362 "2020-06-16T05:40:25Z")\
**Posts on this page:** 3\
**Page:** 1

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**Author:** ![claudiameadows](https://yyz2.discourse-cdn.com/free1/user_avatar/es.discourse.group/claudiameadows/32/126_2.png) [@claudiameadows](https://es.discourse.group/u/claudiameadows)\
**Post date:** [June 16, 2020, 5:40am UTC](https://es.discourse.group/t/batch-copy-to-array-seeking-champion/362/1 "2020-06-16T05:40:25Z")

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See: [https://github.com/isiahmeadows/proposal-array-set](https://github.com/isiahmeadows/proposal-array-set)

We already have a useful `typedArray.set(array, offset = 0)`, but I've run into quite a few glitches with that, cases where an end index would be incredibly useful and cases where I don't want to start at the beginning of `array`. However, that's just a minor nice-to-have to avoid an allocation. The bigger issue I've been running into is that there's no equivalent with arrays, and I'd say at least 1% of all my `for` loops that require indices (1 in 100 is a _lot_) could be outright replaced with this, with another of 2-3% having at least some use for it. (In peformance-critical code, this number ends up even more inflated, with as much as 5% of all loops being literally just this.) `Array.prototype.copyWithin` already exists, so I suspect it's as much a spec omission as anything, and in particular, the parameter order and handling proposed there is the same as that method. (The general intuition with my proposal is that `array.copyWithin(...args)` should be equivalent to `array.set(array, ...args)` in most cases.)

Seeking champion(s) and/or other involvement. No polyfills (yet), of course, though polyfills would be very straightforward to write.

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**Author:** ![michael](https://yyz2.discourse-cdn.com/free1/user_avatar/es.discourse.group/michael/32/11_2.png) [@michael](https://es.discourse.group/u/michael)\
**Post date:** [June 18, 2020, 9:50pm UTC](https://es.discourse.group/t/batch-copy-to-array-seeking-champion/362/2 "2020-06-18T21:50:57Z")

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Can't you do this with Array.prototype.splice?

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<div class="post-metadata">

**Author:** ![claudiameadows](https://yyz2.discourse-cdn.com/free1/user_avatar/es.discourse.group/claudiameadows/32/126_2.png) [@claudiameadows](https://es.discourse.group/u/claudiameadows)\
**Post date:** [June 18, 2020, 11:32pm UTC](https://es.discourse.group/t/batch-copy-to-array-seeking-champion/362/3 "2020-06-18T23:32:31Z")

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In theory you could:

```javascript
// array.set(other, offset)
array.splice(offset, other.length, ...other)

// array.set(other, offset, start, end)
var temp = other.slice(start, end)
array.splice(offset, temp.length, ...temp)

```

But there's a few issues with that:

1. No implementation optimizes for it. (In fact, no implementation optimizes away argument spreads in general, even for obvious cases like `array.push(...other)`.) Instead, `other` gets copied to the full arguments list, which is untenable for all but small arrays.
2. It returns an entire array of all the removed values when returning, which is almost never used. (If you really need that, you can literally just do `array.slice(index, index + other.length)` beforehand.)
3. Copying just a portion of an array to another is a surprisingly common need in my experience, and the above idiom is _very_ expensive as it makes not one, but four different copies: one to slice, one to spread, one to save for the result, and one to store.

And since this occurs primarily in perf-sensitive locations, `splice` is just not practical for my use case. Even if hardware acceleration is used, it would still be slower unless your CPU supports SIMD with over 4 lanes for the pointer width of your addresses (not the case on any mobile CPU), `memmove` is smart enough to use it, and the engine is smart enough to use `memmove` (not the case in any engine AFAICT).

And keep in mind, this is something `splice` and friends could efficiently be built on - it's a lower-level primitive.