Generators and Iterators

A generator function can pause and resume, yielding a sequence of values one at a time instead of computing and returning them all at once.

function* countTo(n) {
    for (let i = 1; i <= n; i++) {
        yield i;
    }
}

for (const number of countTo(3)) {
    console.log(number);
}

Expected output

1
2
3

Generators are what power for...of's ability to iterate arrays, strings, and maps under the hood, and are useful for representing a sequence too large (or infinite) to build as an array upfront.