Ecma262: Most implementations seem to have missed that `await x ** 2` is not legal

Created on 3 Oct 2020 · 7Comments · Source: tc39/ecma262

Moving discussion from https://github.com/microsoft/TypeScript/issues/40916.

Quoting @DanielRosenwasser:

I can't seem to find a derivation for ExponentiationExpression to await 1 ** 1.
ExponentiationExpression seems to need an UpdateExpression on the left, which does not include an AwaitExpression. On the other hand, UnaryExpression does, but that's kind of the point of the syntax split.

My reading (now that I look) matches his. But V8, SpiderMonkey, JavaScriptCore, engine262, babel, shift, flow, and acorn all seem to have missed this. (But not TypeScript or XS!) We should probably call it out with a big note in the spec, or, since it might now be web reality, possibly change the spec to match implementations.

Source

bakkot

👀4

Most helpful comment

As an extra step, this proves that each browser is parsing it as (await X) ** 2 and not await (X ** 2):

(async function f() {
  const obj = {
    valueOf() {
      console.log('Parsed as `await (X ** 2)`');
      return 1;
    },
    then(f) {
      console.log('Parsed as `(await X) ** 2`');
      f(2);
    },
  };
  const value = await obj ** 2;
  return value;
})()

jridgewell on 3 Oct 2020

👍5 👀3

All 7 comments

Incidentally I'd be happy with making this legal, since await a + 2 definitely is.

bakkot on 3 Oct 2020

👍3

My vote is more to keep things as is and fix implementations if possible, as it's more inconsistent otherwise.

DanielRosenwasser on 3 Oct 2020

👍2

As an extra step, this proves that each browser is parsing it as (await X) ** 2 and not await (X ** 2):

(async function f() {
  const obj = {
    valueOf() {
      console.log('Parsed as `await (X ** 2)`');
      return 1;
    },
    then(f) {
      console.log('Parsed as `(await X) ** 2`');
      f(2);
    },
  };
  const value = await obj ** 2;
  return value;
})()

jridgewell on 3 Oct 2020

👍5 👀3

Does it the same order for "await x + y" (await x) + y not await (x + y)?