JS: Array.prototype.includes

Array.prototype.includes

xArray.includes(searchElement)

• Return true if searchElement is in xArray.
• Else, false.

console.log([3, 4, 5].includes(4));
// true

xArray.includes(searchElement, fromIndex)

Begin search at fromIndex.

console.log([3, 4, 5].includes(3, 1) === false);
// true

// negative index, search start at end
console.log([3, 4, 5].includes(3, -1) === false);
// true

Example

On NaN

JS: NaN

console.log([3, NaN, 5].includes(NaN));

On undefined

JS: undefined

console.log([3, undefined, 5].includes(undefined));

On Sparse Array

JS: Sparse Array

const xx = [3, 4];
// make it a sparse array
xx.length = 100;

// includes finds non-existent element
console.log(xx.includes(undefined));
// true

What's the difference between includes vs indexOf

The includes method differs from Array.prototype.indexOf in two ways.

• includes uses the SameValueZero algorithm for equality test, instead of Strict Equality Comparison, allowing it to detect NaN.
• includes does not skip missing array elements, instead treating them as undefined.