do-not-add-custom-methods-to-primitive-objects.md

title	category	date	topics
Do not add custom methods to primitive objects	Best practice	2021-02-22 22:54:00 +7	JavaScript

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'
}