JavaScript Loops: A Step-By-Step Guide


The JavaScript loop is an incredibly useful tool that allows programmers to repeat a certain task a number of times. It gives you the ability to process sets of data within your applications. Simply put, JavaScript loops make code more efficient.

With the ability to repeat code using loops, you can save time instead of writing out similar code each time it’s needed. One of the most powerful JavaScript features that allow you to easily create interactivity and do complex processing is loops.

Types of JavaScript Loops

  • Entry Controlled Loops

An entry controlled loop is a control or iteration statement that executes until something happens, relatively called a guard condition. It is the thing that is being checked before entering the loop. It’s used when you have the need to check the conditions before entering into the actual body of the loop. For and while loops are entry-controlled loops.

  • Exit Controlled Loops

An exit controlled loop is a type of loop where the program will repeat a set of code a certain number of times. When the program reaches the end of the code, it checks to see if there are any conditions that prevent it from ending the loop. If no condition is met, it ends the loop. Otherwise, it will continue to execute. Do while loop is an exit controlled loop.

JavaScript Loops

JavaScript supports the following kinds of loops:

  1. for – A for loop is a programming statement that allows you to repeat a block of code.
    1. for/in – The for/in loop steps through the properties of an object and is a great choice when you have a known, finite list.
    2. for/of – It loops through the values of an iterable object.
  2. while – The JavaScript while statement allows us to loop through a block of code while a specified condition is true. The condition is checked before the loop starts; if the condition is false the first time, there will be no loop at all.
    1. do/while – The do/while loop is a type of loop in JavaScript that runs a specified block of code once before checking a condition, and then runs it over and over until the condition evaluates to false. The do/while loop is similar to the while loop with one major difference. The do/while loop executes the code at least once even if the condition evaluates to false.

1. For Loop

We can perform the same task repeatedly using a for loop. The JavaScript for loop is quite popular among programmers as it makes coding easier. As the name suggests, you need to define a task(s) at once and it can be performed repeatedly using a for loop. The for loop in Javascript is normally used when the number of iterations is known.

JavaScript for loop syntax:

for (statement 1; statement 2; statement 3) {

  // code block to be executed


For/of and For/in Loops in JavaScript

One of the most underused features in JavaScript is for loops. Even though most developers know they exist and probably use them on occasion, most people still don’t really know what they do or how they work. for loops serve a similar purpose to while loops but allow us to loop through an array and perform some sort of operation against each element inside them.

We can also use for/in and for/of loops with arrays and objects to loop through every property and read their values and compare them with certain conditions.

  • for/in Syntax
for (variable in array) {

 // code block to be executed

  • for/of Syntax
for (variable of iterable) {

  // code block to be executed


2. While Loop

The JavaScript while loop is used when the number of iterations required is not known. The first thing worth noting is that the while loop, unlike the for loop, does not include an initialization. Nonetheless, similar to for loop, the while loop will continue execution until the condition turns false.

A variation of the while loop is the do while loop. In the do while loop, the code to be executed is defined first followed by the condition. This allows the loop to run once even if the condition gets false on the first iteration.

  • while Syntax
while (condition) {

  // code block to be executed

  • do/while Syntax
do {

  // code block to be executed


while (condition);


JavaScript loops are integral to coding in JS. They will help you save time for code that requires execution over and over. Also, working with loops will help you better understand the control flow in programming.

Thus, in order to become a better JS developer, you need to have a good understanding of looping in JavaScript. There are so many ways to loop the same code using different variations of for and while loops. However, you need to choose the variation that is the most efficient.

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