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JavaScript - How to Use The Array map() Method in JavaScript

Array map() is a method included in the Array.prototype property which was introduced in ECMAScript 5 (ES5) and is supported in all modern browsers.

Map is an Iteration Method, and like other such methods – forEach(), filter(), reduce() etc. – it helps us iterate through a dataset, applying the specified actions to each element.

Basic map() usage example

Using map() allows us to iterate through all items in an array, performing actions on each item. The results of this activity are returned to us inside a new array.

For example, the following code iterates through an array of numbers and multiplies each number by 2:

const numArray = [6, 7, 15, 22, 45];
const numArrayMult = numArray.map(num => num*2);
console.log(numArrayMult); //Output: [12, 14, 30, 44, 90]
console.log(numArray); //Output: [6, 7, 15, 22, 45]

(This example uses abbreviated syntax, more complex version will be exemplified below the Syntax)

In the example above, which uses the abbreviated map() syntax, numArray was not changed at all. The new array (numArrayMult) has the same length (5) as the input array, and all items were manipulated in the same manner (i.e. the value of each is item * 2).

map() vs. forEach(), filter() and reduce()

When comparing map() versus forEach(), filter(), and reduce(), it is important to note that while all of these methods iterate through an array, they differ in their return value:

  • map() returns a new array with the same length as the parent array.
  • forEach() returns undefined, preventing us from chaining methods together.
  • filter() returns an array containing items that meet a certain condition (e.g. greater than 5). Given this, the filter() method may return an array with the same length, an empty array, or something in between. The final length of the return value depends on how many items met the required condition.
  • reduce() is used to reduce an array to a single value. For example you can use reduce() to sum all of the values of the array, or to retrieve the highest value in the array. Use reduce() whenever it is crucial that you work with a single value built from the underlying array.

Syntax

The syntax of the map() method is straightforward. The method accepts a callback function, which takes the currentValue, index, and the return array as arguments, providing a new array as output:

array.map(callback function (currentValue, index, array), this.Arg)

Consider the following code:

const numArray = [6, 7, 15, 22, 45];
const numArrayMult = numArray.map(function numMultIdx(num, idx) {
  return num*idx;
})
console.log(numArrayMult); // Output: [0, 7, 30, 66, 180]

Let’s explore the code above and compare it to the prototype function:

  • The callback function in this code is numMultIdx
  • The currentValue is each number in the iterated array (numArray), provided in order, and is sent to numMultIdx in the parameter num.
  • Idx simply receives the index of the current item in the iterated array that is being processed.
  • The final argument, array, is the array we are looping through, and is rarely used.

Note: manipulating this.Arg allows the this context to be changed.

Rendering lists in JavaScript libraries

A common usage of the map() method in JavaScript is to render lists of items to the browser. Below is a simple React component that makes use of map() to render a list of books:

import React from 'react';
export function BookList(props) {
  const {books} = props;
  return (
    <section>
      {books.map((book, idx) => {
        return <div book={book} key={idx}></div>  
      })}
    </section>
  )  
}

We can ignore the React syntax for the time being (you can read more about React here) – the important part of this code is within the <section> tags. As you can see, the code iterates through the list of books provided to the map() function, returning a <div> element for each book that has a key attribute equal to the item’s index value.

 

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