fix corner case in awaits

[alexlamsl]

fixes #5157

[kzc]

Is AST_Yield subject to any similar return optimizations?

[alexlamsl]

IIRC the analogue of await-on-async would be yield *-on-generator, as yield would not expand on its value.

console.log(function*() {
    try {
        return yield* function*() {
            throw "FAIL";
        }();
    } catch (e) {
        return "PASS";
    }
}().next().value);

$ cat test.js | node
PASS
$ uglifyjs test.js -mc | node
PASS

[kzc]

IIRC the analogue of await-on-async would be yield *-on-generator, as yield would not expand on its value.

Yes, indeed.

esbuild is useful in understanding and debugging async/await in that regard...

$ cat a.js
async function fail() { 
    throw new Error("PASS");
} 
async function test() { 
    try { 
        return await fail(); 
    } catch (e) { 
        console.log(e.message);
    } 
}
test();

$ cat a.js | esbuild --target=es6
var __async = (__this, __arguments, generator) => {
  return new Promise((resolve, reject) => {
    var fulfilled = (value) => {
      try {
        step(generator.next(value));
      } catch (e) {
        reject(e);
      }
    };
    var rejected = (value) => {
      try {
        step(generator.throw(value));
      } catch (e) {
        reject(e);
      }
    };
    var step = (x) => x.done ? resolve(x.value) : Promise.resolve(x.value).then(fulfilled, rejected);
    step((generator = generator.apply(__this, __arguments)).next());
  });
};
function fail() {
  return __async(this, null, function* () {
    throw new Error("PASS");
  });
}
function test() {
  return __async(this, null, function* () {
    try {
      return yield fail();
    } catch (e) {
      console.log(e.message);
    }
  });
}
test();