Array Map Explained

map() method on an array produces a new array by applying a function to every element of the input array. The result is a new array with the same length of the original input array. map() takes a callback function with arguments. First argument is the current element of the array being processed.

map() is helpful in situations where you need to do some work on every element of the array. Maybe you need to double every element.

const arr = [2, 4, 9, 22];
const map = arr.map(x => x * 2);
console.log(map); // [ 4, 8, 18, 44 ]

You can also access the current index of the array using a map. The callback function takes a second argument for index.

const arr = [2, 4, 9, 22];
const map = arr.map((value, index) => value * index); // multiplying array value with its index
console.log(map); // [ 0, 4, 18, 66 ]

And if access to index is not enough, you can also get access to the original array as a third parameter.

map method and this keyword

There are times when you need to specify the this context in map(). Passing this is supported as a second parameter to map.

const util = {
    firstName: "John",
    lastNames: ["Wick", "Malcolm", "Smith"],
    randomNameGenerator: function () {
        const newNames = this.lastNames.map(function (surname) {
            return (`${this.firstName} ${surname}`); // we can access first name provided by this object
        }, this); // passing reference to this object
        return newNames;
    }
}

console.log(util.randomNameGenerator()); // [ 'John Wick', 'John Malcolm', 'John Smith' ]

When not to use a map

• If you do not plan to use the returned array or if you are not returning a value from the callback function, do not use a map. Use forEach or for..of if you want to update the input array itself.

• Please do not use it with built-in methods that take one parameter most of the time but do have the capability of taking more than one parameters. Let me illustrate with an example.
  

const arr = ["2", "4", "9"];
const map = arr.map(parseInt);
console.log(map);

You might expect the answer as [2, 4, 9] and each element as a number due to the use of parseInt. You would be surprised that the output is [2, NaN, NaN]. Keep in mind that parseInt takes three parameters: element, index and array! So for the second and third element, the index will be 1 and 2 resulting in NaN from parseInt.