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'
}