Array.prototype.includes

This replaces a common pattern when checking an array includes a value.

var arr = [1, 2, 3];
if (arr.indexOf(1) !== -1) {
    console.log('array has 1');
}

This can now be refactored to...

var arr = [1, 2, 3];
if (arr.includes(1)) {
    console.log('array has 1');
}

Why not 'includes' like string?

The proposal was originally for Array.prototype.contains. MooTools extended the native array prototype, causing a conflict. Due to the huge number of sites still using it, it was decided to use includes and rename String.prototype.contains to String.prototype.includes.