Transform [].forEach => for-in

[kristoferbaxter]

Is your feature request related to a problem? Please describe.

Many [].forEach calls can be converted to a for...in safely.

const LANGUAGES = { javascript, json, xml };
Object.keys(LANGUAGES).forEach( key => hljs.registerLanguage(key, LANGUAGES[key]) );

Can be converted to...

const LANGUAGES = { javascript, json, xml };
for (key in LANGUAGES) { hljs.registerLanguage(key, LANGUAGES[key]) }

Describe the solution you'd like

This transform can be done safely so long as the inner function doesn't rely on this being auto bound.

It's worth investigating if there are cases where a function call can be generated that passes the scope as an argument and leverages it correctly as well.

Describe alternatives you've considered

Ignore this transform entirely.

Edit: If order is important, for...of is a better choice.

Because the order of iteration is implementation-dependent, iterating over an array may not visit elements in a consistent order. Therefore, it is better to use a for loop with a numeric index (or Array.prototype.forEach() or the for...of loop) when iterating over arrays where the order of access is important.

[kristoferbaxter]

Note: Can create a unique key for all for ... of and leverage it in each instance.

x.forEach(v=>add(v)); => for(z of x){add(z)}

Pros: Shorter consistent syntax.
Cons: Global variable lookup, hoisted module locals are cheapest to assign/read.

Alternative: Create a local per module for this and let variable folding keep it terse.

let z;
for(z of x){add(z)}

Caveat: Nested loops!