The Complete Guide to Loops in JavaScript

The Complete Guide to Loops in JavaScript

The Complete Guide to Loops in JavaScript

I know there are plenty of places online (StackOverflow, MDN, Morioh) that can be an excellent resource for exploring loops and Array iterable methods. But… sometimes it’s good to write about something just so that these patterns settle in your mind and the knowledge is cemented for good. As a result, I found some things I didn’t know existed. In this article I’ll share them with anyone who needs to get a better insight into loops!

The primary purpose of a loop is to iterate over one or a set of multiple statements. Iterating is a commonplace act in software development. To iterate simply means to repeat an action multiple times.

Types of loops in JavaScript

There are different ways to loop and iterate in JavaScript.

for, for…of, for…in, while, Array.forEach, Array.***** (some of the Array methods are similar to loops / iterators:Array.values(), Array.keys(),, Array.reducer(),etc_._)

for loops

Here’s a visualization of a basic for loop. The blue dots are the statements that will run for a predetermined number of times. (5 times in this case.)

For loops usually start counting from 0.


For loop syntax comes in three syntactic flavors:

for (initialize; condition; increment);
for (initialize; condition; increment) single statement;
for (initialize; condition; increment) { multiple; statements; }

For loops require 3 statements separated by two semicolons, which can be any legal JavaScript statement, a function call, or even an empty statement.

In most cases you’re likely to see the following pattern 1) initialize counter, 2) test condition and 3) increment / decrement counter.

0-index based counter

For loop counters are 0-index based. Let’s skip the middle statement and try breaking out of the loop using our own condition (i > 1):

for (let i = 0;; i++) {
    console.log("loop, i = " + i);
    if (i > 1)

Console output:

"loop, i = 0"
"loop, i = 1"
"loop, i = 2"

The word “loop.” is printed 3 times. The condition (i > 1) might deceive you into thinking that the text will be printed 2 times at most.

But it’s printed 3 times! Because counting started with 0 and (i > 1) = 2.

The Infinite for Loop

A for loop can be defined without any of the default statements. But by doing this you will create an infinite for-loop that will freeze your program:

for(;;) console.log("hi"); // Infinite for loop - don't do it

Multiple Statements

Multiple statements can be separated by commas. In the following example, the inc() function is used to increment the value of a global variable counter. Note the combination of the the two statements: i++, inc():

let counter = 0;
function inc() { counter++; }

for (let i = 0; i < 10; i++, inc());
console.log(counter); // 10

Increment Numbers

Loops are often used to increment numbers.

let counter = 0;
for (let i = 0; i < 10; i++)

counter; // 10;

for loops and let scope.

Bracket-less for-loop syntax is not good friends with the let keyword. The following code will generate an error:

for (var i = 0; i < 10; i++) let x = i;

A let variable cannot be defined without scope brackets. All variables defined using the let keyword require their own local scope. This is fixed by:

for (var i = 0; i < 10; i++) { let x = i; }

Nested for Loops

Because a for loop is a JavaScript statement in itself, it can be used as the iterable statement of another for loop. This hierarchical for loop is often used for working with 2-dimensional grids:

for (let y = 0; y < 2; y++)
for (let x = 0; x < 2; x++)
console.log(x, y);

Console output (all combinations between x / y):

0 0
1 0
0 1
1 1

Loop’s Length

The condition statement will usually check if counter has reached a limit and if so, the loop will terminate:

for (let i = 0; i < 3; i++) console.log(“loop.”);

This simple loop will print “loop.” to console 3 times:


You can add {} brackets if you need to execute multiple statements:

for (let i = 0; i < 3; i++) { let loop = "loop."; console.log(loop) };

The console output will be the same as the previous example.

Skipping Steps

You can skip an iteration step by using the continue keyword:

for (let i = 0; i < 3; i++) { if (i == 1) continue; console.log( i ); }

Output of this for-loop: (note 1 was skipped)


The continue keyword tells code flow to go to the next step without executing any next statements in this for-loop’s scope during the current iteration step.

Breaking Early

You can break out of a for loop by using the break keyword:

for (let i = 0;; i++) { console.log("loop"); break; };

Note the condition statement was skipped here. The loop will break by an explicit command from the statement itself.

In this case the for loop will print “loop.” to the console only once. The break keyword simply exits the loop whenever it’s encountered. But it can also be conditional (see next example.)

Breaking To A Label

In JavaScript, a statement can be labeled when a label_name: is prepended to a statement. Because a for loop is a statement you can label for loops.

Let’s try to increment value of c inside the inner loop. By choosing whether to break the loop and jump to the inner or mark label, we change the pattern in which the for loops will work:

1. This example produces 11 when breaking to mark:

let c = 0;
mark: for (let i = 0; i < 5; i++)
inner: for (let j = 0; j < 5; j++)
c++; if (i == 2) break mark;
console.log(c); // 11

2. This example produces 21 when breaking to inner:

let c = 0;
mark: for (let i = 0; i < 5; i++)
inner: for (let j = 0; j < 5; j++)
c++; if (i == 2) break inner;
console.log(c); // 21

The two examples are logically different.

Breaking from a labeled non for-loop scope

While we’re on the subject, you can use the break keyword to break out of regular non for-loop block scope as long as it’s labeled:

block: {
break block;

Console output:


Execution flow never reaches “after”.

for…of Loop

Using indexed iterators can become a hassle when dealing with arrays or object properties. Especially when their number is not known.

for…of loops to the rescue!

for…of Loops And Generators

Sometimes you might want to use a for loop with a generator.

Generator executes a yield statement in an asynchronous way but it will execute only one next yield statement every time the function is called:

// Generator function:
function* generator() {
yield 1;
yield 2;
yield 3;

for (let value of generator())
console.log( value );

Console output:


for…of Loops And Strings

Strings are iterable.

You can walk each character of a string using a for…of loop:

let string = 'text';

for (let value of string)
console.log( value );



for…of Loops And Arrays

Let’s say we have an array:

let array = [0, 1, 2];

We can iterate through it without having to create index variables. Once the end of the array is reached the loop will end automatically:

for (let value of array)
console.log( value );

Console output:


for…of Loops And Objects (won’t work)

It would be nice to iterate over object properties too, right?

let object = { a: 1, b: 2, c: 3 };

for (let value of object) // Error: object is not iterable
console.log( value );

But for…of loops work only with iterable values. An object is not an iterable.

for…of loops and objects that were converted to iterables

As a remedy you can first convert an object to an iterable using some of the built-in Object methods: .keys().values() or .entries():

let enumerable = { property : 1, method : () => {} };

for (let key of Object.keys( enumerable )) console.log(key);
> property
> method

for (let value of Object.values( enumerable )) console.log(value);
> 1
> () => {}

for (let entry of Object.entries( enumerable )) console.log(entry);
> (2) ["prop", 1]
> (2) ["meth", ƒ()]

This is also achievable by using a for…in loop instead without Object.*

for…in Loops

The for…of loops (in the previous section) only accept iterables.

for…in loops and enumerable object properties

When you have an object at hand you should probably use a for…in loop.

A for…in loop will display only enumerable object properties:

let object = { a: 1, b: 2, c: 3, method: () => { } };

for (let value in object)
console.log(value, object[value]);

Output: (Note that methods are enumerable too.)

() => { }

Non-enumerable properties are hidden from for…in

You won’t see constructor and prototype properties in an output from the for…in loop. Although they exist on an object they are not considered to be enumerable.

While Loops

A while loop will iterate for an indefinite number of times until the specified condition (there is only one) evaluates to false. At which point your loop will stop and execution flow will resume.

while (condition) { /* do something until condition is false */ }

Once condition evaluates to false the while loop will break automatically:

let c = 0;

while (c++ < 5)

Console output:


A secondary condition can be tested within the loop. This makes it possible to break from the loop earlier if needed:

while (condition_1) {
if (condition_2)

While and continue

The continue keyword can be used to skip steps:

let c = 0;
while (c++ < 1000) {
if (c > 1)

Console output:



Many of the Array.* methods are iterators. Instead of passing your array into a for or a while loop you should use built-in Array methods instead. Arrays usually already have methods offering cleaner syntax for anything you would write yourself to solve the same problem. So why re-invent the wheel?

Array methods are attached to Array.prototype property.This means you can execute them directly from array object like array.forEach() or directly from array’s literal value like: [1,2,3].forEach();


Return value: none

The forEach method will execute a function for every item in the array.

Each iteration step receives 3 arguments value, index, object.

It’s similar to a for-loop but it looks cleaner:

let fruit = ['pear', 'banana', 'orange', 'apple', 'pineapple'];
let print = function(item, index, object) { console.log(item); }
fruit.forEach( print );

Starting from ES6 it can be suggested to use arrow functions together with Array methods. The code will be easier to read and maintain when building large scale applications. Let’s take a look at how we can make syntax cleaner:

1.) Because in JavaScript functions are also expressions, you can pass the function directly into the forEach method:

fruit.forEach(function(item, index, object) {
console.log(item, index, object);

2.) You might want to use an arrow function: () => {}

fruit.forEach((item, index, object) => {
console.log(item, index, object);

The console output from both of the cases above:

"pear",      0, (5)["pear","banana","orange","apple","pineapple"]
"banana", 1, (5)["pear","banana","orange","apple","pineapple"]
"orange", 2, (5)["pear","banana","orange","apple","pineapple"]
"apple", 3, (5)["pear","banana","orange","apple","pineapple"]
"pineapple", 4, (5)["pear","banana","orange","apple","pineapple"]

3.) If you can get away with one argument and return statement use:

fruit.forEach(item => console.log(item));

As long as you have only one single statement, you can remove {} brackets.



Return value: boolean

Not to be confused with the “execute for every item” logic of forEach. In many cases method every will actually not run on every item in the array when at least one item doesn’t evaluate to true based on specified condition.

The method every will return true if the value of every single item in the array satisfies the condition specified in its function argument:

let numbers = [0,1,2,3,4,5,6,7];
let result = numbers.every(value => value < 10 );
result; // true

The result is true because none of the numbers in the array are greater than or equal to 10. Let’s take at the same function with a different value set. If 10 or a greater number was present in the array the result would be false:

let numbers = [0,1,256,3,4,5,6,7];
let result = numbers.every(value => value < 10 );
result; // false

Here one of the numbers is 256. Which can be translated to “not every value in the array is < 10”. Hence, false is returned. It’s important to note that once Array.every method encounters 256 the condition function will not execute on the remaining items. Just a single failed test will cause false.

Array.every does not modify the original array. The value inside the function is a copy, not a reference to the value in the original array:

let numbers = [0,1,256,3,4,5,6,7];
let result = numbers.every(value => value++ < 10 );
console.log(result); // false
console.log(numbers); // Original array is unchanged


Return value: boolean

Similar to Array.every except it stops looping whenever it encounters a value that evaluates to true (and not false like in Array.every) Let’s compare:

let numbers = [0,10,2,3,4,5,6,7];
let condition = value => value < 10; // value is less than 10
let some = numbers.some(condition); // true
let every = numbers.every(condition); // false

Here, some returns true because it checks first value: 0 for < 10 and immediately returns true without having to check the rest of the values.

The every method returns false on the same data set. That’s because when it reaches the second item whose value is 10, the < 10 test fails.

Note: Try not to think of some as an “opposite” of every. In some cases they return the same result on the same data set.


Return value: new array consisting only of items that passed a condition.

let numbers = [0,10,2,3,4,5,6,7];
let condition = value => value < 10;
let filtered = numbers.filter(condition);
console.log(filtered); // [0,2,3,4,5,6,7] // 10 is filtered out!
console.log(numbers); // Original array remains unchanged

The new filtered array contains all original items except 10. Because it did not pass the < 10 test. In a real-world scenario the condition can be much more complex and involve larger objects sets.

Return value: a copy of the original array with modified values (if any.)

let numbers = [0,1,256,3,4,5,6,7];
let result = => value = value + 1 );
console.log(result); // [1,2,257,4,5,6,7,8] // incremented by 1
console.log(numbers); // Original array is unchanged is like Array.forEach but it returns a copy of the modified array. Note that the original array is still unchanged.


Return value: accumulator

Reducers are similar to other methods. Yet they are unique because they have an accumulator value. The accumulator value must be initialized. There are different types of reducers. In this first example, we’ll take a simple case.

As values are iterated, this accumulator adds all numbers into a single value:

const R = (accumulator, currentValue) => accumulator + currentValue;

Like any other Array method that works with iterables, a reducer has access to the value it is currently iterating (currentValue).

const numbers = [1, 2, 4];
const R = (accumulator, currentValue) => accumulator + currentValue;
console.log(numbers.reduce(R)); // 7

This reducer added up all the numbers into the single accumulator value and returned it: 1 + 2 + 4 = 7.

How to understand reducers in more complex, practical situations?

When developing software in the real world you won’t be using reducers to count numbers. This can be done with a simple for loop. You will encounter plenty of situations where a set of data should be “reduced” only to the set of important values based on some criteria.

Array.reduce or Array.filter?

When it comes to Array methods, always try to to choose a proper tool for the task. Don’t use reduce just because you want to use reduce. You’ll notice that some things you plan to use reduce for can be done with filter.

However, one of the most common use cases for a reducer is updating the application view and/or properties of a database entry, via some API call.

Reducers And Updating Object Properties In A Database

After a database action round trip, you may want to update the application view. But why update all objects everywhere, when you can “reduce” which object properties should be affected, without having to copy the entire object?

Let’s say your car listing management application has a button that updates the price of a particular vehicle. The user sets a new price and clicks on the button. An action is dispatched to update the vehicle in the database. Then the callback function returns containing the object with all properties for that vehicle ID. But, we only need to update the price. A reducer can make sure to update only the price not the entire object. The object is then sent back to the database and the application view is updated.

Creating Your Own Object Iterables With [Symbol.iterator]

In some advanced cases you might be interested in this pattern for creating your own iterable object. To be honest though I can’t think of anything practical. If you know anything about this or have practical experience with Symbol.iterator, post a comment and perhaps then I can update the tutorial.

let iterable = {
Symbol.iterator {
return {
i : 0,
next() {
if (this.i < 3)
return { value : this.i++, done : false }
return { value : 1, done : true }

for (let value of iterable)
console.log( value );

Learn More

Introducing TensorFlow.js: Machine Learning in Javascript

5 ways to build real-time apps with JavaScript

Full Stack Developers: Everything You Need to Know

ES5 to ESNext — here’s every feature added to JavaScript since 2015

5 Javascript (ES6+) features that you should be using in 2019

The Complete JavaScript Course 2019: Build Real Projects!

JavaScript: Understanding the Weird Parts

Vue JS 2 - The Complete Guide (incl. Vue Router & Vuex)

The Full JavaScript & ES6 Tutorial - (including ES7 & React)

Thanks for reading!

Originally published by JavaScript Teacher at

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JavaScript developers should you be using Web Workers?

JavaScript developers should you be using Web Workers?

Do you think JavaScript developers should be making more use of Web Workers to shift execution off of the main thread?

Originally published by David Gilbertson at

So, Web Workers. Those wonderful little critters that allow us to execute JavaScript off the main thread.

Also known as “no, you’re thinking of Service Workers”.

Photo by Caleb Jones on Unsplash

Before I get into the meat of the article, please sit for a lesson in how computers work:

Understood? Good.

For the red/green colourblind, let me explain. While a CPU is doing one thing, it can’t be doing another thing, which means you can’t sort a big array while a user scrolls the screen.

This is bad, if you have a big array and users with fingers.

Enter, Web Workers. These split open the atomic concept of a ‘CPU’ and allow us to think in terms of threads. We can use one thread to handle user-facing work like touch events and rendering the UI, and different threads to carry out all other work.

Check that out, the main thread is green the whole way through, ready to receive and respond to the gentle caress of a user.

You’re excited (I can tell), if we only have UI code on the main thread and all other code can go in a worker, things are going to be amazing (said the way Oprah would say it).

But cool your jets for just a moment, because websites are mostly about the UI — it’s why we have screens. And a lot of a user’s interactions with your site will be tapping on the screen, waiting for a response, reading, tapping, looking, reading, and so on.

So we can’t just say “here’s some JS that takes 20ms to run, chuck it on a thread”, we must think about where that execution time exists in the user’s world of tap, read, look, read, tap…

I like to boil this down to one specific question:

Is the user waiting anyway?

Imagine we have created some sort of git-repository-hosting website that shows all sorts of things about a repository. We have a cool feature called ‘issues’. A user can even click an ‘issues’ tab in our website to see a list of all issues relating to the repository. Groundbreaking!

When our users click this issues tab, the site is going to fetch the issue data, process it in some way — perhaps sort, or format dates, or work out which icon to show — then render the UI.

Inside the user’s computer, that’ll look exactly like this.

Look at that processing stage, locking up the main thread even though it has nothing to do with the UI! That’s terrible, in theory.

But think about what the human is actually doing at this point. They’re waiting for the common trio of network/process/render; just sittin’ around with less to do than the Bolivian Navy.

Because we care about our users, we show a loading indicator to let them know we’ve received their request and are working on it — putting the human in a ‘waiting’ state. Let’s add that to the diagram.

Now that we have a human in the picture, we can mix in a Web Worker and think about the impact it will have on their life:


First thing to note is that we’re not doing anything in parallel. We need the data from the network before we process it, and we need to process the data before we can render the UI. The elapsed time doesn’t change.

(BTW, the time involved in moving data to a Web Worker and back is negligible: 1ms per 100 KB is a decent rule of thumb.)

So we can move work off the main thread and have a page that is responsive during that time, but to what end? If our user is sitting there looking at a spinner for 600ms, have we enriched their experience by having a responsive screen for the middle third?


I’ve fudged these diagrams a little bit to make them the gorgeous specimens of graphic design that they are, but they’re not really to scale.

When responding to a user request, you’ll find that the network and DOM-manipulating part of any given task take much, much longer than the pure-JS data processing part.

I saw an article recently making the case that updating a Redux store was a good candidate for Web Workers because it’s not UI work (and non-UI work doesn’t belong on the main thread).

Chucking the data processing over to a worker thread sounds sensible, but the idea struck me as a little, umm, academic.

First, let’s split instances of ‘updating a store’ into two categories:

  1. Updating a store in response to a user interaction, then updating the UI in response to the data change
  2. Not that first one

If the first scenario, a user taps a button on the screen — perhaps to change the sort order of a list. The store updates, and this results in a re-rendering of the DOM (since that’s the point of a store).

Let me just delete one thing from the previous diagram:

In my experience, it is rare that the store-updating step goes beyond a few dozen milliseconds, and is generally followed by ten times that in DOM updating, layout, and paint. If I’ve got a site that’s taking longer than this, I’d be asking questions about why I have so much data in the browser and so much DOM, rather than on which thread I should do my processing.

So the question we’re faced with is the same one from above: the user tapped something on the screen, we’re going to work on that request for hopefully less than a second, why would we want to make the screen responsive during that time?

OK what about the second scenario, where a store update isn’t in response to a user interaction? Performing an auto-save, for example — there’s nothing more annoying than an app becoming unresponsive doing something you didn’t ask it to do.

Actually there’s heaps of things more annoying than that. Teens, for example.

Anyhoo, if you’re doing an auto-save and taking 100ms to process data client-side before sending it off to a server, then you should absolutely use a Web Worker.

In fact, any ‘background’ task that the user hasn’t asked for, or isn’t waiting for, is a good candidate for moving to a Web Worker.

The matter of value

Complexity is expensive, and implementing Web Workers ain’t cheap.

If you’re using a bundler — and you are — you’ll have a lot of reading to do, and probably npm packages to install. If you’ve got a create-react-app app, prepare to eject (and put aside two days twice a year to update 30 different packages when the next version of Babel/Redux/React/ESLint comes out).

Also, if you want to share anything fancier than plain data between a worker and the main thread you’ve got some more reading to do (comlink is your friend).

What I’m getting at is this: if the benefit is real, but minimal, then you’ve gotta ask if there’s something else you could spend a day or two on with a greater benefit to your users.

This thinking is true of everything, of course, but I’ve found that Web Workers have a particularly poor benefit-to-effort ratio.

Hey David, why you hate Web Workers so bad?

Good question.

This is a doweling jig:

I own a doweling jig. I love my doweling jig. If I need to drill a hole into the end of a piece of wood and ensure that it’s perfectly perpendicular to the surface, I use my doweling jig.

But I don’t use it to eat breakfast. For that I use a spoon.

Four years ago I was working on some fancy animations. They looked slick on a fast device, but janky on a slow one. So I wrote fireball-js, which executes a rudimentary performance benchmark on the user’s device and returns a score, allowing me to run my animations only on devices that would render them smoothly.

Where’s the best spot to run some CPU intensive code that the user didn’t request? On a different thread, of course. A Web Worker was the correct tool for the job.

Fast forward to 2019 and you’ll find me writing a routing algorithm for a mapping application. This requires parsing a big fat GeoJSON map into a collection of nodes and edges, to be used when a user asks for directions. The processing isn’t in response to a user request and the user isn’t waiting on it. And so, a Web Worker is the correct tool for the job.

It was only when doing this that it dawned on me: in the intervening quartet of years, I have seen exactly zero other instances where Web Workers would have improved the user experience.

Contrast this with a recent resurgence in Web Worker wonderment, and combine that contrast with the fact that I couldn’t think of anything else to write about, then concatenate that combined contrast with my contrarian character and you’ve got yourself a blog post telling you that maybe Web Workers are a teeny-tiny bit overhyped.

Thanks for reading

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Further reading

An Introduction to Web Workers

JavaScript Web Workers: A Beginner’s Guide

Using Web Workers to Real-time Processing

How to use Web Workers in Angular app

Using Web Workers with Angular CLI