Javascript Array map()

Introduction

Javascript array map() method creates a new array by calling mapping function on each element. 

let new_array = arr.map(function callback( currentValue[, index[, array]]) {
}[, thisArg])
Parameter
 Optional
 Meaning
callback




 Not




 function called for every element.
The callback function takes three arguments:
currentValue - the current element being processed.
index - Optional, the index of the current element.
array - Optional, the array itself.
thisArg
 Optional
 value to use as this when executing callback.

It is not called for missing elements of the array.

The following code calculates the square roots one each array element. 

let numbers = [1, 4, 9]
let roots = numbers.map(function(num) {
    return Math.sqrt(num)
})
console.log(numbers);/*from   w  w  w.  jav  a 2s  .com*/
console.log(roots);

Using map to reformat objects in an array 

let kvArray = [{key: 1, value: 10},  
               {key: 2, value: 20},  /*from  ww  w  .  java2 s  . c  om*/
               {key: 3, value: 30}]

let reformattedArray = kvArray.map(obj => {
     let rObj = {};
     rObj[obj.key] = obj.value+1;
     return rObj;
})

Double each element in the array 

let numbers = [1, 4, 9]
let doubles = numbers.map(function(num) {
  return num * 2/*w  w w .j av  a  2  s.com*/
})
console.log(numbers);
console.log(doubles);

Get the full name for each person in the array: 

var persons = [//from w w w .j a va 2  s. c om
  {firstname : "A", lastname: "X"},
  {firstname : "B", lastname: "Y"},
  {firstname : "C", lastname: "Z"}
];


function getFullName(item) {
  var fullname = [item.firstname,item.lastname].join(" ");
  return fullname;
}

console.log(persons.map(getFullName));



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