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The anatomy of a for loop in vanilla JS (and when you would want to use it instead of Array.forEach())

Today, I wanted to go back to basics and take a look at the humble for loop.

What a for loop does

A for loop runs code over and over again until a condition is met.

It’s broken up into three parts, each separated by a semicolon (;):

  1. Before the first semicolon, you can declare or assign variables.
  2. Between the first and second semicolon, you define a condition to check after each loop. As long as this condition is true, the loop keeps running. Once the condition is false, the loop stops.
  3. After the second semicolon, you can specify a statement to run after each loop.

For example, let’s say you wanted to log the numbers 1 through 10 into the console. Once you get to 10, you want to stop.

You would defined an initial index variable called i (this can be anything you want, but i is commonly used), and give it a value of 1. For your conditional check, you’ll make sure that i is less than 11. As long as that’s true, the loop will keep running.

After each loop, you’ll add 1 to i using the increment operator (++). In between the curly brackets, you’ll log i to the console.

for (var i = 1; i < 11; i++) {

Here’s a demo.

Looping through array items

Let’s say you had an array of wizard names, and wanted to loop through each name and log it to the console.

var wizards = ['Harry', 'Hermione', 'Neville', 'Ron', 'Dumbledore'];

To do this, you would define a variable i and give it a value of 0. For your conditional check, you would check to make sure i is less than the length of the wizards array. After each loop, you’ll add 1 to i.

Inside the loop, you can use i to get the item at that index in the wizards array and log it to the console.

for (var i = 0; i < wizards.length; i++) {

See it in action here.

Ending and skipping for loops

Inside the loop, you can use the continue operator to skip the current item in the loop and move on to the next one. You can use the break operator to end the loop early.

Looking at our wizards array again, let’s imagine you wanted to log every wizard except Harry.

You would check to see if wizards[i] is "Harry", and if so, use the continue operator.

for (var i = 0; i < wizards.length; i++) {
	if (wizards[i] === 'Harry') continue;

If you wanted to end the loop if the wizard is Ron, you would use break.

for (var i = 0; i < wizards.length; i++) {
	if (wizards[i] === 'Harry') continue;
	if (wizards[i] === 'Ron') break;

Play with skipping and ending loops here.

When and why would you use a for loop instead of Array.forEach()?

For looping over arrays, Array.forEach() provides a much nicer syntax. But sometimes, a for loop is still a better choice.

If you want to end a loop early if a certain condition is met, there’s no way to do that with Array.forEach(). In that case, for is a better option.

The for loop is also for more than just looping over arrays. If you need to run a loop a certain number of times (for example, to automatically generate an array of items), use a for loop.

Browser Compatibility

The for loop has been around forever. It works in all modern browsers, and back to IE 3 (yes, that’s right, 3!).