ForEach, Map, Filter and Reduce in Javascript

ForEach, Map, reduce, and filter

hese all are 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.

ex:
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.

ex:
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.

ex:
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;
};

ForEach, Map, Filter and Reduce in Javascript