Iterator.prototype.some()

callbackFn

A function to execute for each element produced by the iterator. It should return a truthy value to indicate the element passes the test, 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.

true if the callback function returns a truthy value for at least one element. Otherwise, false.

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

The main advantage of iterator helpers over array methods is that they are lazy, meaning that they only produce the next value when requested. This avoids unnecessary computation and also allows them to be used with infinite iterators. With infinite iterators, some() returns true as soon as the first truthy value 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().some(isEven)); // true

const isNegative = (x) => x < 0;
const isPositive = (x) => x > 0;
console.log(fibonacci().take(10).some(isPositive)); // false
console.log(fibonacci().some(isNegative)); // Never completes

Calling some() 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.some(isEven)); // true
console.log(seq.next()); // { value: undefined, done: true }

• Polyfill of Iterator.prototype.some in core-js
• Iterator
• Iterator.prototype.every()
• Iterator.prototype.find()
• Array.prototype.some()