Manual using generator with yield without await · Issue #117 · tc39/proposal-async-iteration (original) (raw)

I try code below in babel-node, and Chrome Canary(v64), and get different result
Unfortunately I could not understand what behavior is correct by this spec

In this example I call iterator.next() twice, and synchronously

1. I expect that point#2 and task#2 will be called immediately(within second iterator.next()), because there is no await for the task#1
   I give this behavior at babel-node, but not in Chrome Canary
   Chrome execute point#2 only when task#1 resolved, like as I wrote yield await task('task#1') instead yield task('task#1')
2. Should the promiseCapability/IteratorResult be resolved only when the result is resolved, and contain resolved value?
   2.1) In babel-node IteratorResult resolved, when yield got the value(pending Promise), and return this value without additional awaiting.
   In other word IteratorResult..value can be a Promise
   2.2) In Chrome IteratorResult resolved, when yield got the value(pending Promise), and this value also resolved.
   In other word IteratorResult..value cannot be a Promise - it is always fulfilled value

I read "Async Generator Rewrite", and it turns out that the behavior of the babel is correct
But when I read #114, I'm confused

What is correct behavior for this example?

const start = Date.now(), iterator = iteratorBar(); logIteratorResult('next#1', iterator.next()); logIteratorResult('next#2', iterator.next());

async function* iteratorBar() { console.log(time(), 'point#1'); yield task('task#1'); console.log(time(), 'point#2'); yield task('task#2'); }

async function task(value) { console.log(time(), ${value} - started); return new Promise((resolve) => setTimeout(resolve, 1000, value)); }

async function logIteratorResult(name, iteratorResult) { let {value, done} = await iteratorResult; console.log(time(), ${name} IteratorResult - resolved, value); value = await value; console.log(time(), ${name} IteratorResult.value - resolved, value); }

function time() { return Date.now() - start; }

babel-node execution log

1 'point#1'
3 'task#1 - started'
4 'point#2'
4 'task#2 - started'
34 'next#1 IteratorResult - resolved' Promise { <pending> }
34 'next#2 IteratorResult - resolved' Promise { <pending> }
1004 'next#1 IteratorResult.value - resolved' 'task#1'
1004 'next#2 IteratorResult.value - resolved' 'task#2'

Chrome Canary execution log

1 "point#1"
2 "task#1 - started"
1002 "point#2"
1002 "task#2 - started"
1003 "next#1 IteratorResult - resolved" "task#1"
1003 "next#1 IteratorResult.value - resolved" "task#1"
2003 "next#2 IteratorResult - resolved" "task#2"
2004 "next#2 IteratorResult.value - resolved" "task#2"