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Iterator.prototype.includes() - JavaScript | MDN

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Syntax

js
includes(searchElement)
includes(searchElement, fromIndex)

Parameters

searchElement

The value to search for.

fromIndex Optional

Zero-based index at which to start searching. Must be a non-negative integer, Infinity, or undefined. If fromIndex is greater than or equal to the number of elements produced by the iterator (including when fromIndex is Infinity), the method always returns false after exhausting the iterator. If fromIndex is undefined, it defaults to 0.

Return value

A boolean value which is true if the value searchElement is found within the iterator (or the part of the iterator starting at fromIndex, if specified).

Exceptions

TypeError

Thrown if fromIndex is not one of: an integer, Infinity, -Infinity, or undefined.

RangeError

Thrown if fromIndex is negative.

Description

The includes() method compares searchElement to elements of the array using the SameValueZero algorithm. This algorithm works like strict equality === (where -0 and +0 are considered equal), with the exception that NaN is considered equal to itself.

Unlike Array.prototype.includes(), the fromIndex parameter of Iterator.prototype.includes() is not allowed to be negative, because the iterator does not have a known length. The type validation is also stricter: non-integer values are not truncated to integers.

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, includes() returns true as soon as the first match is found. If the value is never encountered, the method never returns.

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

Examples

Using includes()

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

console.log(fibonacci().includes(8)); // true
console.log(fibonacci().take(10).includes(7)); // false
console.log(fibonacci().includes(7)); // Never completes

The method closes the iterator after returning.

js
const seq = fibonacci();
console.log(seq.includes(8)); // true
console.log(seq.next()); // { value: undefined, done: true }

Using fromIndex

fromIndex specifies the number of elements to be skipped from the beginning. It is equivalent to calling drop(fromIndex).includes(searchElement).

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

console.log(fibonacci().includes(8, 3)); // true
console.log(fibonacci().includes(8, 10)); // false

If fromIndex is greater than or equal to the elements available, false is returned.

["a", "b", "c"].values().includes("a", 3); // false