Why does this async generator cause the JavaScript runtime to hang?

Ben :

The following JavaScript causes the runtime to hang on Chrome (v80.0.3987.116) and Firefox (v72.0.2) on OSX 10.15.2.

Why?

Note that I am marking the iterator function as async.

const iterable = {
    async *[Symbol.iterator]() {
        yield 'one'
    }
}

console.log([...iterable])

Bergi :

Because now your [Symbol.iterator] method no longer returns an Iterator, it does return an AsyncIterator. That's still an object with a .next() method, so the spread syntax trying to iterate it doesn't complain. But the calls to the .next() method do no longer return {value:…, done:true} objects, they always return a promise. These promise objects have no truthy done property, so your iterator never stops…

You can achieve the same result with

const iterable = {
    *[Symbol.iterator]() {
        while (true) {
            yield
        }
    }
}    
console.log([...iterable])

or

const iterable = {
    [Symbol.iterator]() {
        return {
            next() {
                return {};
            }
        }
    }
}
console.log([...iterable])