[CS2] Optimize do (...args) -> blocks to use let

[dead-claudia]

Blocks with let implicitly create closures, so you could optimize non-global do blocks to use let rather than IIFEs to generate smaller, cleaner code.

[connec]

@isiahmeadows do you have some example current vs. desirable output?

[vendethiel]

We need to make sure do (i) still works the same.

[dead-claudia]

@vendethiel That was implied. I was just referring to do (i) -> ...

[GeoffreyBooth]

Can you please provide examples?

[dead-claudia]

Here's an example:

# CoffeeScript
res = do (i = 0) ->
    while i < len
        i += 1 if someCondition((s) -> i < s)
        i += 1
    i

// Current JS
var res;
res = (function (i) {
    while (i < len) {
        if (someCondition(function (s) { return i < s })) i += 1;
        i += 1;
    }
    return i;
})(0);

// Proposed JS
var res;
{
    let i = 0;
    while (i < len) {
        if (someCondition(function (s) { return i < s })) i += 1;
        i += 1;
    }
    res = i;
}

(I know the ability to expand functions like that exists, because of how if/else and for already expand similarly and recursively in non-returning locations).

[GeoffreyBooth]

Basically this is a refactor of do that drops the function scope, and uses block scope instead with variables declared by let to achieve the same effect. Sure, I guess. It’s more idiomatic ES, I suppose, but it’s not like the current JS is so unreadable.

If you’d like to implement this as a PR I suppose it would be an improvement. I’m not going to keep the issue open though because I don’t think this is something the maintainers would prioritize for one of us to tackle; though if you want to give it a shot, by all means please submit a PR.

[connec]

I think this is pretty interesting, especially in the context of #2518.

Something to consider though is that do can also be used for function application:

something = -> alert 'woah'
do something