[plugin-transform-for-of] Ability to opt in or out of for-of loose mode

[trusktr]

Feature Request

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

Loose mode is nice, even though my target environments support for-of, I'd like to keep using loose mode to have faster iteration on arrays with convenient for-of syntax.

But sometimes, in rare cases, I may want to take advantage of native for-of in order to edit arrays in place while iterating (including adding or deleting items), but this doesn't work in loose mode for obvious reasons.

Describe the solution you'd like

It'd be nice to have a comment that can enable (or disable) loose for-of. F.e. If we're in loose mode, we could write the following to enable strict mode (it may compile, or may simply output the for-of as-is depending on env config):

// @babel no-loose
for (const item of array) {
  // ...
}

Not sure about the comment format, but you get the idea.

Conversely, someone with loose mode turned off might want to do

// @babel loose
for (const item of array) {
  // ...
}

The no-* could be a generic commenting mechanism to disable/enable some feature.

Describe alternatives you've considered

I can opt for the strict mode by converting an array into a Set, for example. But it seems clunky.

Teachability, Documentation, Adoption, Migration Strategy

...

---

Sidenote, in my tests, the following hand-written version I've been using,

var arr = [1, 2, 3];
for (let i = 0, l = arr.length; i < l; i+=1) {
  var a = arr[i];
}

is almost three times faster than Babel's output.

var _arr = [1, 2, 3];
for (var _i = 0; _i < _arr.length; _i++) {
  var a = _arr[_i];
}

[babel-bot]

Hey @trusktr! We really appreciate you taking the time to report an issue. The collaborators
on this project attempt to help as many people as possible, but we're a limited number of volunteers,
so it's possible this won't be addressed swiftly.

If you need any help, or just have general Babel or JavaScript questions, we have a vibrant Slack
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[trusktr]

Here's a test that shows the speed difference:

var arr = Array.from({length: 1000000});

var n = 0
var start = performance.now()
for (var i = 0; i < arr.length; i++) {
  n+=1
}

console.log('test 1 time:', performance.now() - start)

var n = 0
var start = performance.now()
for (var i = 0, l = arr.length; i < l; i+=1) {
  n+=1
}

console.log('test 2 time:', performance.now() - start)

// Output:
// test 1 time: 8.255000007920898
// test 2 time: 2.970000001369044