What’s the yield keyword in JavaScript?

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Late answering, probably everybody knows about yield now, but some better documentation has come along.

Adapting an example from “Javascript’s Future: Generators” by James Long for the official Harmony standard:

function * foo(x) {
    while (true) {
        x = x * 2;
        yield x;
    }
}

“When you call foo, you get back a Generator object which has a next
method.”

var g = foo(2);
g.next(); // -> 4
g.next(); // -> 8
g.next(); // -> 16

So yield is kind of like return: you get something back. return x returns the value of x, but yield x returns a function, which gives you a method to iterate toward the next value. Useful if you have a potentially memory intensive procedure that you might want to interrupt during the iteration.

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