Iterator.prototype.every()

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 callbackFn returns a truthy value for every element. Otherwise, false.

every() iterates the iterator and invokes the callbackFn function once for each element. It returns false immediately if the callback function returns a falsy value. Otherwise, it iterates until the end of the iterator and returns true. If every() returns false, 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, every() returns false as soon as the first falsy value is found. If the callbackFn always returns a truthy 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().every(isEven)); // false

const isPositive = (x) => x > 0;
console.log(fibonacci().take(10).every(isPositive)); // true
console.log(fibonacci().every(isPositive)); // Never completes

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

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