Understand Map, Filter and Reduce in JavaScript

Understand Map, Filter and Reduce in JavaScript

If you’re starting in JavaScript, maybe you haven’t heard of .map(), .reduce(), and .filter(). For me, it took a while as I had to support Internet Explorer 8 until a couple years ago.

When I learned how to use .map(), .filter(), and .reduce() everything I read, watched, and heard sounded so complicated. These concepts were being taught as standalone implementations that I couldn’t wrap my head around.

I heard that these were gateway concepts symbolizing an ascension into enlightenment. I wish I had been told the truth: That these three methods are recognition—and implementation—that the reason you loop through an iterable often falls into one of three functional categories.

Reviewing my previously written code, I realized that 95% of the time when looping through strings or arrays I do one of the following: map a sequence of statements to each value, filter values that meet a specific criteria, or reduce the data set to a single aggregate value.

This was my breakthrough moment. Map, filter, and reduce each simply perform one of these tasks!

To practice, I took my old code and refactored it using these methods. That was supremely helpful.

So without further ado, let’s get on with it. We’ll explain each method then convert common for loop implementations to their respective method.


The .map() method is used when you want to 1. perform a set of statements with every value in the iterable and 2. return the (presumably) modified value.

Let’s use a simple example of calculating sales tax on an array of prices.

const prices = [19.99, 4.95, 25, 3.50];
let new_prices = [];
for(let i=0; i < prices.length; i++) {
   new_prices.push(prices[i] * 1.06);

We can achieve the same results using .map():

const prices = [19.99, 4.95, 25, 3.50];
let new_prices = prices.map(price => price * 1.06);

The syntax above is condensed so let’s walk through it a bit. The .map() method takes a callback, which can be thought of as a function. That’s what is between the parentheses.

The variable price is the name that will be used to identify each value. Since there’s only one input, we can omit the usual parentheses around the parameters.

The statement after the arrow => is the body of our callback. Since the body has only one statement, we can omit the curly braces as well as the return keyword.

Just in case this is still confusing, let’s write it out fully for reference:

const prices = [19.99, 4.95, 25, 3.50];
let new_prices = prices.map((price) => {
   return price * 1.06

On to the .filter() method, which is used when you want to extract a subset of values from the iterable. When using .filter(), remember that we are filtering in values, not filtering out. This means that each item in the iterable that evaluates true will be included in the filter.

Let’s use an example of keeping only odd integers. We’re using the modulus operator to calculate the remainder of dividing by 2. When that remainder equals 1, we know the number was odd.

const numbers = [1,2,3,4,5,6,7,8];
let odds = [];
for(let i=0; i < numbers.length; i++) {
   if(numbers[i] % 2 == 1) {

Similar to the .map() method, .filter() accepts a single callback to where each value in the iterable will be passed.

const numbers = [1,2,3,4,5,6,7,8];
let odds = numbers.filter(num => num % 2);

Similar rules apply for this callback. Since there’s a single input, and the body of the function is a single expression, we can omit the parameter list parentheses, the curly braces defining the body, and the return keyword.


Finally, we come to .reduce(), which, admittedly, is the most confusing of the three methods. The name of the method refers to reducing multiple values to one. However, I found that it’s easier to think of it as building up rather than reducing.

The method works by defining a starting point. As the method iterates over each value, that starting point is modified and passed down.

Here’s a classic case of adding a series of numbers together. Pretend we’re calculating the total donations to your favorite charity:

const donations = [5, 20, 100, 80, 75];
let total = 0;
for(let i=0; i < donations.length; i++) {
   total += donations[i];

Unlike .map() and .filter(), the .reduce() method’s callback requires two parameters: an accumulator and the current value. The accumulator will be the first parameter and is the “pass it down” value.

const donations = [5, 20, 100, 80, 75];
let total = donations.reduce((total,donation) => {
   return total + donation;

We could also pass a second argument to the .reduce() method itself. This would serve as the starting value for the accumulator. Let’s say we’re adding yesterday’s donations that totaled $450.

const donations = [5, 20, 100, 80, 75];
let total = donations.reduce((total,donation) => {
   return total + donation;
}, 450);

There you have it. These methods are not scary! Think of them as making your code more readable. You are writing more condensed code, but more importantly, you are actually describing the intention of your loop.

You’ll have a much easier time reading your code when you look back at it in three months. Instead of having to read the statements within the for loop, just to understand its high-level intention, you can see map/filter/reduce and begin to have an idea about what the block is trying to achieve.

Angular 9 Tutorial: Learn to Build a CRUD Angular App Quickly

What's new in Bootstrap 5 and when Bootstrap 5 release date?

What’s new in HTML6

How to Build Progressive Web Apps (PWA) using Angular 9

What is new features in Javascript ES2020 ECMAScript 2020

Programming a Javascript Simon Game Tutorial

Programming a Javascript Simon Game Tutorial

In this javascript tutorial, I recorded myself live programming an html5 javascript simon game.

In this javascript tutorial, I recorded myself live programming an html5 javascript simon game.

For those who don't know, I'm a full stack web developer who has been in the industry for over 5 years now. There is a lot of things I have learned along the way and I'd like to share that knowledge with anyone wanting to learn!

like this video if you found it useful and would like to see more videos of the same content.

subscribe to my channel if you are trying to improve your abilities as a web developer, software engineer, or even if you are just learning to code.

Don't forget to turn on those bell notifications!

Understanding Memoization And Dynamic Programming in Javascript

Understanding Memoization And Dynamic Programming in Javascript

In this Javascript tutorial I will explain what memoization is, how to use it, when you should use memoization, how to use memoization, what dynamic programming is, how to use memoization in dynamic programming. Memoization is a big complicated word that you may have never even heard before, but you may be surprised to know that you are most likely already using memoization without even realizing it.

Memoization is a big complicated word that you may have never even heard before, but you may be surprised to know that you are most likely already using memoization without even realizing it. Memoization is just the act of caching values so that they can be calculated quicker in the future. Memoization is really useful in all parts of programming, but where it is most useful is in dynamic programming. In this video I will explain what memoization is, how to use it, and why it is so useful especially in dynamic programming.

🧠 Concepts Covered:

  • What memoization is
  • When you should use memoization
  • How to use memoization
  • What dynamic programming is
  • How to use memoization in dynamic programming

JavaScript Programming Tutorial - Full JavaScript Course for Beginners

JavaScript Programming Tutorial - Full JavaScript Course for Beginners

JavaScript is the programming language of HTML and the Web. JavaScript is easy to learn. This tutorial will teach you JavaScript from basic to advanced. JavaScript Programming Tutorial - Full JavaScript Course for Beginners

Watch this JavaScript tutorial for beginners to learn JavaScript programming from scratch. 👍

⭐️Course Contents⭐️

01 | Introduction

Get an introduction from Bob, as he kicks off the course, sets expectations, and gives you a little guidance for the road ahead.

02 | Setting Up the Development Environment

See how to install Node and Visual Studio Code, and learn why you need each. Watch as Bob writes a first tiny line of code to make sure that the installation is ready.

03 | Basic JavaScript Syntax

Hear about the basics of the JavaScript syntax, including how JavaScript is executed, how to create a properly formed statement, and more.

04 | Variables

Learn what variables are and how to declare, initialize, set, and get values from them.

05 | Data Types

JavaScript works with data types differently than other programming languages do. Learn from Bob as he explains some of the most basic data types and how to determine the type held by a variable.

06 | Type Coercion and Conversion

Sometimes you need to explicitly convert the data type of a value into a different data type. Sometimes JavaScript will do this automatically for you. Bob explains how and why this happens.

07 | Expressions and Operators

Learn how to spot a well-formed expression and how to compose expressions through the combination of operators and operands.

08 | Arrays

Watch as Bob demonstrates how to use arrays, how to add/remove elements of an array, how to access elements of an array using the index, how to get the number of elements in an array, and more.

09 | Function Declaration

Get an introduction to functions, as Bob shows you one way to define a function, how to call the function, how to pass parameters into a function, how to return values from a function, and more.

10 | Function Expressions

Sometimes a function declaration is more than you need. Get details in this lesson, as Bob demonstrates how to use a function expression and how to create an immediately invoked function expression.

11 | Decision Statements

Learn how to add logic to your applications by testing conditions using if / else statements, switch / case statements, and the ternary operator.

12 | Iteration Statements

Watch as Bob introduces the For and While statements, which enable you to loop through lists of values and perform operations on them.

13 | Basics of Scope

Hear an explanation of the notion of the lifetime of variables and their reach into other code blocks, in this discussion of scope.

14 | Returning Functions from Functions

As a building block toward a more sophisticated approach to building JavaScript applications, see how you can return functions from functions.

15 | Object Literals

Objects contain properties and functions that describe attributes of the object and its behavior, respectively. Watch as Bob demonstrates how to create object literals.

16 | Module Pattern and Revealing Module Pattern

Learn how to introduce variables and functions without leaving an unnecessarily large footprint on the global scope.

17 | Closures

Closures allow developers to bind functions to their execution context, including variables, so you can build specialized versions of functions. Learn how closures work and how to create them.

18 | this Keyword

Get the details on the "this" keyword, which causes a lot of confusion for JavaScript developers. Listen as Bob explains what this mysterious object is and how to bend it to your will.

19 | Destructuring

See how to use destructuring, a new feature in JavaScript that allows you to unpack array elements or object properties in a concise syntax.

20 | String Template Literals

Learn about the new string template literal syntax to enable string interpolation, multi-line string formatting, and more.

21 | Regular Expressions

Working with string data, Bob explains how use regular expressions to determine such things as whether a string matches a specific pattern or whether an instance of the pattern exists in a string.

22 | Built-In Natives

Join Bob as he examines native, built-in JavaScript functions that return objects that box the primitive types in order to provide additional helper methods on your values.

23 | Constructor Function Calls with the new Keyword

Learn how constructor functions work, see how to make key distinctions in what they are, and find out why they’re helpful.

24 | Objects and the Prototype Chain

Moving past the absolute beginner matter, listen in as Bob explains how objects work in JavaScript and how you can create an object that is linked to another object.

25 | JavaScript Classes

Bob demonstrates how “syntactic sugar” works on top of the existing functions and prototype chaining to help JavaScript resemble more traditional object-oriented programming languages.

26 | Arrow Functions

The latest version of JavaScript added arrow functions, a shorthand syntax for creating functions. Watch as Bob demonstrates how to create them and shows where they’re useful.

27 | Truthy and Falsy Values

Listen is as Bob circles back to topics that are important (but didn’t easily fit into earlier topics) and looks at the odd way in which JavaScript evaluates certain expressions as truthy and falsy.

28 | null Type

Similar to the undefined primitive type, the null type represents a variable that has no reference to an object when one was expected. Hear an explanation of how and why we should think about nulls.

29 | Date Objects

No discussion of JavaScript would be complete without understanding the Date built-in native function which provides properties and methods for working with dates and times.

30 | String Methods

The built-in native String function provides several very useful methods for manipulating and evaluating strings. Watch Bob demonstrate how to use a few of the most useful ones.

31 | Array Methods

We’ve worked with arrays throughout this course. Now Bob takes a few minutes to show you some of the most useful methods provided by the Array built-in native function.

32 | Error Handling with Try Catch

See how to wrap a try/catch/finally statement around potentially problematic code to anticipate everything that could go wrong with it, and learn other error-handling tips.

33 | Understanding the Document Object Model

Watch Bob examine the web browser environment and how it creates an object graph of nodes that represent elements, their attributes, and text, plus how to properly attach your code to an HTML page.

34 | Working with DOM Nodes

Continuing from the previous video, Bob demonstrates a slightly more compelling example that helps us understand how to manipulate, create, and delete element nodes and attribute nodes.

35 | Course Conclusion

In this final video, Bob briefly adds some closing comments and well wishes. You can also optionally complete a survey to provide insight into how Microsoft can improve this course.

Thanks for watching

If you liked this post, please do share/like it with all of your programming buddies!

Follow us on Facebook | Twitter