JSFiddle - React, Tailwind, and code Playground

JavaScript

// add a method conditionally
if (!('method' in Array.prototype)) {
    Array.prototype.method = function (name, func) {
        if (!this[name]) this[name] = func;
    }
}

var arr = [1,2];

// exclusive push
arr.method('xadd', function (value) {
    if (this.indexOf(value) === -1) {
        this.push(value)
    };
});

arr.xadd(3);
console.log(arr)