JSFiddle - React, Tailwind, and code Playground

JavaScript

Array.prototype.myFunc = function () {
    console.log("calling myFunc");
};

Object.defineProperty(Array.prototype, "myMethod", {
    value: function () {
        console.log("calling myMethod");
    },
    enumerable: false
});

var a = [1, 2, 3];
for (var key in a) {
    console.log("Normal enumeration: key=" + key);
}

for (var key in a) {
    var msg = "hasOwnProperty enumeration: key=";
    var isOwnProperty = a.hasOwnProperty(key);
    console.log(msg + key + ", isOwnProperty=" + isOwnProperty);
}

console.log("All keys of `a`: " + Object.keys(a));