Map(), filter() and reduce() in javasCript

 Map()

       JavaScript map() is a built-in function that allows you to process and transform all the items in a iterable without using an explicit for loop, a technique commonly known as mapping.

    map() is useful when you need to apply a transformation function to each item in an iterable and transform them into a new iterable.

// double the all array items 
const arr = [5,1,2,3,4];
function double(x){
    return x * 2
}
const output = arr.map(double)
console.log(output)

[ 10, 2, 4, 6, 8 ]

Using Arrow function

const arr = [5, 1, 2, 3, 4];
const output = arr.map(x => x * 2)
console.log(output)

Filter()

    JavaScript filter() is aa built-in function that allows you to process an iterable and extract those items that satisfy a given condition.

    This process is commonly known as a filtering operation. with filter() you can apply a filtering function to an iterable and produce new  iterable with the items that satisfy that condition at hand. 

// sepreate Even numbers 
const arr = [5, 1, 2, 3, 4];
function even(x) {
    return x % 2 == 0
}
const output = arr.filter(even);
console.log(output)

[ 2, 4 ]

Using Arrow function

// sepreate Even numbers 
const arr = [5, 1, 2, 3, 4];
const output = arr.filter(x => x % 2 == 0);
console.log(output)

reduce() 

    JavaScript reduce() is a function that implements a mathematical technique called folding or reduction.

    reduce() is useful when you need to apply a function to an iterable and reduce it to a single cumulation value.

 Ex.

//sum of all elements
const arr = [1, 2, 3, 4, 5];
function sum(arr) {
    let sum = 0
    for (let i = 0; i < arr.length; i++) {
        sum = sum + arr[i]
    }
    return sum
}
console.log(sum(arr))

15

Using reduce method.

    The reduce() method take a two argument 1st is a function and 2nd is a initial value. this function take a 2 parameter 1st is a accumulator and 2nd current. current represent each and every value of array. and accumulator represent the initial value.

const arr = [1, 2, 3, 4, 5];
function addition(acc, curr) {
    acc = acc + curr
    return acc
}
const output = arr.reduce(addition, 0);
console.log(output)

using arrow function

const output = arr.reduce((acc, curr) => {
    return acc = acc + curr;
}, 0);
console.log(output)