JavaScript Array map()

let numbers = [2, 4, 6, 8, 10];

// function to return the square of a number
function square(number) {
  return number * number;
}

// apply square() function to each item of the numbers list
let square_numbers = numbers.map(square);
console.log(square_numbers);

// Output: [ 4, 16, 36, 64, 100 ]

map() Syntax

The syntax of the map() method is:

arr.map(callback(currentValue), thisArg)

Here, arr is an array.

map() Parameters

The map() method takes in:

• callback - The function called for every array element. Its return values are added to the new array. It takes in:
  • currentValue - The current element being passed from the array.
• thisArg (optional) - Value to use as this when executing callback. By default, it is undefined.

map() Return Value

• Returns a new array with elements as the return values from the callback function for each element.

Example 1: Mapping array elements using custom function

const prices = [1800, 2000, 3000, 5000, 500, 8000];

let newPrices = prices.map(Math.sqrt);
// [ 42.42640687119285, 44.721359549995796, 54.772255750516614,
//   70.71067811865476, 22.360679774997898, 89.44271909999159 ]
console.log(newPrices);

// custom arrow function
const string = "JavaScript";
const stringArr = string.split(''); // array with individual string character

let asciiArr = stringArr.map(x => x.charCodeAt(0));

// map() does not change the original array
console.log(stringArr); // ['J', 'a', 'v', 'a','S', 'c', 'r', 'i', 'p', 't']

console.log(asciiArr); // [ 74,  97, 118,  97, 83,  99, 114, 105, 112, 116 ]

Output

[
  42.42640687119285,
  44.721359549995796,
  54.772255750516614,
  70.71067811865476,
  22.360679774997898,
  89.44271909999159
]
[
  'J', 'a', 'v', 'a',
  'S', 'c', 'r', 'i',
  'p', 't'
]
[
   74,  97, 118,  97,
   83,  99, 114, 105,
  112, 116
]

Example 2: map() for object elements in array

const employees = [
    { name: "Adam", salary: 5000, bonus: 500, tax: 1000 },
    { name: "Noah", salary: 8000, bonus: 1500, tax: 2500 },
    { name: "Fabiano", salary: 1500, bonus: 500, tax: 200 },
    { name: "Alireza", salary: 4500, bonus: 1000, tax: 900 },
];

// calculate the net amount to be given to the employees
const calcAmt = (obj) => {
    newObj = {};
    newObj.name = obj.name;
    newObj.netEarning = obj.salary + obj.bonus - obj.tax;
    return newObj;
};

let newArr = employees.map(calcAmt);
console.log(newArr);

Output

[
  { name: 'Adam', netEarning: 4500 },
  { name: 'Noah', netEarning: 7000 },
  { name: 'Fabiano', netEarning: 1800 },
  { name: 'Alireza', netEarning: 4600 }
]

Note: map() assigns undefined to the new array if the callback function returns undefined or nothing.

Recommended Reading: JavaScript Array filter()