transforming for await loop not supported for browser that supports it

[thepassle]

I noticed esbuild doesnt transform for-await loops for build target safari11:

✘ [ERROR] Transforming for-await loops to the configured target environment ("safari11") is not supported yet

    app.js:3:6:
      3 │   for await (const i of a) {
        ╵       ~~~~~

However, according to both caniuse and mdn, safari11 does support for-await loops.

It made me wonder if there is a particular reason esbuild doesnt currently handle it that I might be unaware of, or is this an oversight?

Reproduction:
https://stackblitz.com/edit/node-3gwvxg?file=index.js

[evanw]

It made me wonder if there is a particular reason esbuild doesnt currently handle it that I might be unaware of, or is this an oversight?

The reason is that it's a very obscure feature that almost no one uses and it's non-trivial to implement. I think you're the first person to ask about it. In all my time writing JavaScript professionally I've seen exactly one use of for await and it was easily replaced by a for loop containing an await inside it.

However, according to both caniuse and mdn, safari11 does support for-await loops.

My source for browser compatibility data says that they are supported in Safari 12+. I just checked Safari 11.1 on BrowserStack and for await can't even be parsed, so my compatibility data appears to be correct and MDN/Caniuse appears to be wrong. Here's a screenshot:

The simplest workaround for you is probably to just use a for loop with an await in it instead of using a for await loop.

[thepassle]

The reason is that it's a very obscure feature that almost no one uses and it's non-trivial to implement. I think you're the first person to ask about it. In all my time writing JavaScript professionally I've seen exactly one use of for await and it was easily replaced by a for loop containing an await inside it.

Yeah, I definitely agree that the code that uses it could(/should) be refactored to not use a for-await loop, but unfortunately the code is not directly under our maintenance.

Interesting that both caniuse and mdn are off on this one, thanks for confirming that indeed safari11 does not support for await loops.

Thanks for the detailed and swift answer 🙂 I'll close this issue then

[LarsDenBakker]

@evanw would you consider adding an option to let these pass through uncompiled? Then at least the rest of the code could be compiled to a certain target or browser.

[evanw]

Why would you want to do that? Then the code still wouldn't work in the target browser. You could run another tool (e.g. Babel/TypeScript/SWC) after esbuild to post-process the code for the target browser, but then why not have that other tool transform other syntax too? If you want to use esbuild transform away the newest syntax that your other tools don't yet support, you can still use --target=es2018 with esbuild and continue to use for await.