So lets start with Map.

1. Map

Basics:

• Basically it executes a function on each element of an array.

• It returns a new array with the same length.

Use case:

When we want to perform the same operation on each element of array and get back a new array of the same length with the transformed values.

Example:

var numbers = [1,2,3,4,5];
var doubled_arr = numbers.map(n => n * 2);
console.log(doubled_arr);

// output
[2,4,6,8,10]

2. Filter

Basics:

• It executes a function on each element of an array.
• It removes items which don't satisfy a condition and returns filtered array.

Use case:

You have an array and want to filter out certain items. The result is a new array with the same items, but with some excluded.

Example:

var numbers = [1,2,3,4,5];
var greaterThanTwo = numbers.filter(n=> n > 2);
console.log( greaterThanTwo);

// Output
[3,4,5]

3. Reduce

Basics:

• We need to declare the initial value of the accumulator.
• On each iteration, inside the callback we perform some operation and that will be added to the accumulator.
• Returns a single value as output.

Example:

Var numbers = [1,2,3,4,5];
var initialVal = 0;
let result = numbers.reduce(( acc ,val )=> val + acc, initialVal);
console.log(result);

// Output
15