The map function creates a new array populated with the results of the calling function.
const array1 = [1, 4, 9, 16]; // pass a function to map const map1 = array1.map(x => x * 2); console.log(map1); // expected output: Array [2, 8, 18, 32] Syntax : let new_array = arr.map(function callback( currentValue[, index[, array]]) { // return element for new_array }[, thisArg]) callback - a required function which takes in currentValue, index, the array as parameters currentValue - required parameter. This is the current value of the item in the array index - optional parameter. This is the index of the item that is selected array - optional parameter. This is the array on which map function gets executed. the map function is called for each item in the array, such that the result is returned as a new processed item as per requirement.
Mapping an array of numbers to an array of square roots
let numbers = [1, 4, 9] let sum = numbers.map(function(num) { let add = 5; add += num return add; }) // sum is now [6, 9, 14] // numbers is still [1, 4, 9] Using map function to reformat objects in an array
let kvArray = [{key: 1, value: 10}, {key: 2, value: 20}, {key: 3, value: 30}] let reformattedArray = kvArray.map(obj => { let rObj = {} rObj[obj.key] = obj.value return rObj }) // reformattedArray is now [{1: 10}, {2: 20}, {3: 30}], // kvArray is still: // [{key: 1, value: 10}, // {key: 2, value: 20}, // {key: 3, value: 30}] Mapped array contains undefined
let arr = ["We", "Are", "Bros", 4] let filteredArr = numbers.map(function(item, index) { if (index < 3) { return item } }) // index goes from 0, so the filterNumbers are 1,2,3 and undefined. // filteredArr is ["We", "Are", "Bros", undefined] // numbers is still ["We", "Are", "Bros", 4] 
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