Array.prototype.map()

This is a tutorial on map() methods with explanation and examples.

On MDN, the map() method is described as:

"to create a new array populated with the results of calling a >provided function on every element in the calling array."

Basically, it applies the following steps to the original array:

1. iterate through every element in the array
2. apply call back function to each element
3. return the result to a new array without changing the original array.

Let's look at an example:

First let's create a new array with planet name strings:

const solarSystem = ['Mercury', 'Venus', 'Earth', 'Mars', 'Jupiter', 'Saturn', 'Uranus', 'Neptune', 'Pluto'];

And then, we apply .map() method to our solarSystem array to add the string "This is " in front of every element in the array, and save the new array as solarSystem2. This function could be shortened into arrow array.

 const solarSystem2 = solarSystem.map(function(planet){
 return "This is " + planet;
 });

When you print these array

console.log(solarSystem);
console.log(solarSystem2);

You will get:

[
  'Mercury', 'Venus',
  'Earth',   'Mars',
  'Jupiter', 'Saturn',
  'Uranus',  'Neptune',
  'Pluto'
]
[
  'This is Mercury',
  'This is Venus',
  'This is Earth',
  'This is Mars',
  'This is Jupiter',
  'This is Saturn',
  'This is Uranus',
  'This is Pluto'
]

The call back function and the element input to the call back function are compulsory parameters, let's now look at other parameters that you could use in this method:

parameter "index":

const solarSystem3 = solarSystem.map(function(planet, index){
    return "This is " + planet + " number " + (index+1);
});

parameter "array":

const solarSystem4 = solarSystem.map(function(planet, index, originalArray){
    return "This is " + planet + " and the next planet is " + originalArray[index + 1];
});