Add some useful methods to native JavaScript Array

ArrayPollyfill.js

Object.assign(Array.prototype, {
  has(val) {
    return this.indexOf(val) > -1;
  },
  first() {
    return this[0];
  },
  last() {
    return this[this.length - 1];
  }
});

const arr = [1, 2, 3];

console.assert(arr.first() === 1, arr);
console.assert(arr.last() === 3, arr);
console.assert(arr.has(4) === false, arr);
console.assert(arr.has(3) === true, arr);