callbackFn

A function to execute for each element produced by the iterator. It should return a truthy value to make the element yielded by the iterator helper, and a falsy value otherwise. 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 returns the next element in the iterator for which the callback function returns true. 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, filter() allows you to iterate over only those elements that satisfy a given condition.

The following example creates an iterator that yields terms in the Fibonacci sequence, and then reads the first few terms that are even:

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

const seq = fibonacci().filter((x) => x % 2 === 0);
console.log(seq.next().value); // 2
console.log(seq.next().value); // 8
console.log(seq.next().value); // 34

filter() 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().filter((x) => x % 2 === 0)) {
  console.log(n);
  if (n > 30) {
    break;
  }
}

// Logs:
// 2
// 8
// 34

This is equivalent to:

for (const n of fibonacci()) {
  if (n % 2 !== 0) {
    continue;
  }
  console.log(n);
  if (n > 30) {
    break;
  }
}

• Polyfill of Iterator.prototype.filter in core-js
• Iterator
• Iterator.prototype.forEach()
• Iterator.prototype.every()
• Iterator.prototype.map()
• Iterator.prototype.some()
• Iterator.prototype.reduce()
• Array.prototype.filter()