callbackFn

A function to execute for each element produced by the iterator. It should return a truthy value to indicate a matching element has been found, 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.

The first element produced by the iterator that satisfies the provided testing function. Otherwise, undefined is returned.

find() iterates the iterator and invokes the callbackFn function once for each element. It returns the element immediately if the callback function returns a truthy value. Otherwise, it iterates until the end of the iterator and returns undefined. If find() returns an element, the underlying iterator is closed by calling its return() method.

The main advantage of iterator helpers over array methods is their ability to work with infinite iterators. With infinite iterators, find() returns the first satisfying element as soon as it is found. If the callbackFn always returns a falsy value, the method never returns.

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

const isEven = (x) => x % 2 === 0;
console.log(fibonacci().find(isEven)); // 2

const isNegative = (x) => x < 0;
console.log(fibonacci().take(10).find(isNegative)); // undefined
console.log(fibonacci().find(isNegative)); // Never completes

Calling find() always closes the underlying iterator, even if the method early-returns. The iterator is never left in a half-way state.

const seq = fibonacci();
console.log(seq.find(isEven)); // 2
console.log(seq.next()); // { value: undefined, done: true }

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