ForEach, Map, Filter and Reduce in Javascript

ForEach, Map, reduce, and filter are all array methods in JavaScript. Map, reduce, and filter will iterate over an array and perform a transformation or computation and return some value while ForEach is used to just iterate over an array.

ForEach: ForEach method executes a provided function once for each array element.

ex: 
let arrA = [1,2,3,4]
arrA.forEach(element => console.log(element));
// [1,2,3,4]

Map: Execute on each element of array and return new array with same length.

let arrA = [1,2,3,4]
let arrB = arrA.map(_i => i*2)
console.log(arrB)

// [1,4,6,8]

Filter: Filter basically return new array with elements which satisfy condition or filter out the elements which not satisfy the condition.

let arrA = [1,2,3,4]
let arrB = arrA.filter(_i => i%2)
console.log(arrB)

// [2,4]

Reduce: Reduce basically return a single value. While using it we have do define a initial value of accumulator and in each iteration we have to perform some operation and that will be added to accumulator.

let arrA = [1,2,3,4]
let initialVal=0
let result = arrA.reduce((accu, val) => val + accu , initialVal);
console.log(result)

// 10

Polyfill for ForEach, Map, Filter and Reduce

A polyfill is a piece of code (usually JavaScript on the Web) used to provide modern functionality on older browsers that do not natively support it.

Array.prototype.customForEach = function(callback) {
    for (var i = 0; i < this.length; i++)
        callback(this[i], i, this);
};

Array.prototype.customMap = function(callback) {
    arr = [];
    for (var i = 0; i < this.length; i++)
        arr.push(callback(this[i], i, this));
    return arr;
};

Array.prototype.customFilter = function(callback, context) {
    arr = [];
    for (var i = 0; i < this.length; i++) {
        if (callback.call(context, this[i], i, this))
            arr.push(this[i]);
    }
    return arr;
};

Array.prototype.customReduce = function(callback, initialVal) {
    var accum = (initialVal === undefined) ? undefined : initialVal;
    for (var i = 0; i < this.length; i++) {
        if (accum !== undefined){
            accum = callback.call(undefined, accum, this[i], 
                   i, this);
        } else{
            accum = this[i];
        }
    }
    return accum;
};