Is yield* equivalent to a while loop with plain yield in javascript?

Question

If I have a line like:

yield* foo()

Could I replace it with something to the tune of:

while(true) {
 var x = foo.next();
 if(x.done)
 break;
 yield x;
}

Clearly that's more verbose, but I'm trying to understand if yield* is merely syntactic sugar, or if there's some semantic aspect I'm unclear on.

• Felix Kling: Nope, it's pretty much that. Simpler: for (var x of foo()) { yield x; }. Spec: ecma-international.org/ecma-262/7.0/… . edit: Well, you have to call foo() first to get the iterator.
• guest271314: "The yield* expression is used to delegate to another generator or iterable object." developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/…*
• Joshua Engel: Thanks, @FelixKling. I feel myself developing cancer of the semicolon.
• Joshua Engel: And thanks for the spec link, @FelixKling. The specification has some extra features for handling throws from iterables. And there's an oddity in that yield actually yields an IterResultObject that yield* does not.... plus something else about a type called "return".

Answer 1

Instead of yield x you need to do yield x.value. You also need to call foo to get the iterator. .next is a method of the iterator returned by the foo generator.

function *foo() {
 yield 1;
 yield 2;
 yield 3;
}
function *f() {
 yield *foo();
}
console.log(Array.from(f()));
function *g() {
 var iterator = foo();
 while (true) {
 var x = iterator.next();
 if (x.done) break;
 yield x.value;
 }
}
console.log(Array.from(g()));