callbackFn

A function to execute for each element produced by the iterator. Its return value is yielded by the iterator helper. The function is called with the following arguments:

element

The current element being processed.

index

The index of the current element being processed.

A new iterator helper. Each time the iterator helper's next() method is called, it gets the next element from the underlying iterator, applies callbackFn, and yields the return value. When the underlying iterator is completed, the iterator helper is also completed (the next() method produces { value: undefined, done: true }).

The main advantage of iterator helpers over array methods is their ability to work with infinite iterators. With infinite iterators, map() allows you to create a new iterator that, when iterated, produces transformed elements.

The following example creates an iterator that yields terms in the Fibonacci sequence, transforms it into a new sequence with each term squared, and then reads the first few terms:

function* fibonacci() {
  let current = 1;
  let next = 1;
  while (true) {
    yield current;
    [current, next] = [next, current + next];
  }
}

const seq = fibonacci().map((x) => x ** 2);
console.log(seq.next().value); // 1
console.log(seq.next().value); // 1
console.log(seq.next().value); // 4

map() is most convenient when you are not hand-rolling the iterator. Because iterators are also iterable, you can iterate the returned helper with a for...of loop:

for (const n of fibonacci().map((x) => x ** 2)) {
  console.log(n);
  if (n > 30) {
    break;
  }
}

// Logs:
// 1
// 1
// 4
// 9
// 25
// 64

This is equivalent to:

for (const n of fibonacci()) {
  const n2 = n ** 2;
  console.log(n2);
  if (n2 > 30) {
    break;
  }
}

• Polyfill of Iterator.prototype.map in core-js