[feat] Allow use of downlevelIteration on es2015 or greater

Search Terms

downlevelIteration es2015 es6

Suggestion

A new flag or way to enable downlevelIteration when targeting es2015 or greater Maybe forceDownlevelIteration ?

Use Cases

for .. of is slow: https://jsperf.com/for-vs-forof This request comes from my recent work in Nodejs where you want to use higher targets for better performance from the runtime but have to replace for .. of with for due to it’s inefficiencies. Devs shouldn’t be forced to write less ergonomic code for performance reasons when TypeScript could simply do it for you.

An example of having to refactor due to increasing the language target: https://github.com/angular/angular/pull/24534

Examples

N/A

Checklist

My suggestion meets these guidelines:

• This wouldn’t be a breaking change in existing TypeScript / JavaScript code
• This wouldn’t change the runtime behavior of existing JavaScript code
• This could be implemented without emitting different JS based on the types of the expressions
• This isn’t a runtime feature (e.g. new expression-level syntax)

[Toxicable]

@kitsonk Agreed, but as I pointed out above, not everyone knows that most of the other methods of iteration are slow compared to for.

Also writing for loops are much less ergonomic compared to for ... of, I personally would like to use the newer language features without feeling hindered by the performance concerns that go with it.

[alfaproject]

Sounds to me that what you want is a TSLint rule or something along those lines. It’s not TS responsibility to downlevel language features that are natively supported by the target. That goes against its philosophy, surely.