JavaScript: Array.prototype.includes

New in JS2016.

arrayX.includes(searchElement)

Return true if searchElement is in arrayX. Else, false.

arrayX.includes(searchElement, fromIndex)

Begin search at fromIndex.

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.

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

NaN example:

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

undefined example:

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

Sparse Array example:

const arr = [3,4];
arr.length = 100; // make it a sparse array, lots of non-existent elements

// includes will consider non-existent element as undefined
console.log( arr.includes(undefined) ); // true

// indexOf will consider non-existent element as undefined
console.log( arr.indexOf ( undefined ) === -1 ); // true
// it returns -1, which means not found