Add _.slice function

[megawac]

Add a slice function with the python contract _.slice(collection, start, end, step)

See https://docs.python.org/2/library/functions.html#slice. Like python slice, it doesn't do any coercing of inputs (besides 0 which is coerced to 1). Python will give an error for invalid inputs, this implementation doesn't bother and carries on. One nice benefit of the step feature is now we can do non destructive reversing _.slice(arr, null, null, -1)

Created some jsperf benchmarks the other day http://jsperf.com/cr-52768

I think it may make sense to add a _.reverse function after this which will also fix a bug _(document.childNodes).reverse(); // Exception

[jdalton]

As long as the start and end align with Array#slice I'm cool with it.
The addition of step is ok by me too.

it doesn't do any coercing of inputs (besides 0 which is coerced to 1)

???

think it may make sense to add a _.reverse function after this which will also fix a bug

The reverse chaining method is intended for arrays under "Arrays" category so using anything else is undefined territory. I think _.slice may fall into this too as it isn't designed to fix bugs iterating strings in older enviros.

[michaelficarra]

@megawac: please rebase

[megawac]

Rebased

[michaelficarra]

👍

[jashkenas]

Can you explain why and when you'd want to use this instead of vanilla slice?

[jdalton]

The primary reason for me was the vanilla slice can return sparse arrays, the util version will return dense (which is what other Underscore methods return). There's a bonus of small/medium collections being sliced faster, supporting nodelists in IE < 9, and Graeme's addition of the step param too.

[jashkenas]

Are there any use cases for step outside of reversing?

[jridgewell]

http://jsperf.com/cr-52768/6

I think this is trying to do too much. Can we make it a simple, dense version of Array#slice?

[megawac]

@jashkenas from my experience, the step arg is most commonly used for reversing but it can also be useful for picking every 2nd element. I'm not sure if thats useful enough for us to entertain though (was fun to implement though)

[jgonggrijp]

Not all review comments were addressed.