Do not add custom methods to primitive objects

It is not recommended to add a custom method to primitive objects such as `Array`, `Boolean`, `Number`, `String`, etc. Since the `for ... in` statement loops over the enumerable properties, it will include new methods which are added to the prototype.

Array.prototype.isEmpty = function () {
    return (this.length = 0);
};

const a = ['cat', 'dog', 'mouse'];
for (let i in a) {
    console.log(i); // '0', '1', '2', 'isEmpty'
}