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JavaScript Debugging Basics: Part 2

First off, thanks to everyone who responded to yesterday’s debugging challenge. It’s always interesting to see how others approach problems and think about code.

Today, I’m going to walk you through how I would debug an issue like this.

The power of the humble console.log() #

The console.log() let’s you, as the name implies, log data into the console tab in your browser’s developer tools.

It’s incredibly handy for debugging code.

When I encounter a problem like this, I start with the last known working piece of code and work my way through until I find something unexpected.

Debugging our script #

As a quick refresher from yesterday’s article, here’s our event listener:

document.addEventListener('click', function (event) {

    // Make sure clicked element is our toggle
    // To do this, make sure it has the data-toggle attribute
    var toggleId = event.target.getAttribute('data-toggle');

    // If the clicked element doesn't have a data-toggle attribute, bail
    if (!toggleId) return;

    // Prevent default link behavior
    event.preventDefault();

    // Get the content that has the same ID as the data-toggle value
    var content = document.querySelector(toggleId);

    // If no matching element is found, bail
    if (!content) return;

    // Toggle the content
    toggle(content);

}, false);

I started by logging running! in the console immediately after the click event was called to make sure it was working properly. Then I clicked the button.

document.addEventListener('click', function (event) {

    console.log('running!');

    // ...

}, false);

So far, so good.

Next, I logged the toggleId variable to make sure the right element—the close button—was being picked up by the listener, and clicked the button again.

document.addEventListener('click', function (event) {

    // Make sure clicked element is our toggle
    // To do this, make sure it has the data-toggle attribute
    var toggleId = event.target.getAttribute('data-toggle');

    console.log(toggleId);

    // ...

}, false);

As expected, the close button’s [data-toggle] attribute value—#close—was logged. Then, I logged the content variable.

document.addEventListener('click', function (event) {

    // Make sure clicked element is our toggle
    // To do this, make sure it has the data-toggle attribute
    var toggleId = event.target.getAttribute('data-toggle');

    // If the clicked element doesn't have a data-toggle attribute, bail
    if (!toggleId) return;

    // Prevent default link behavior
    event.preventDefault();

    // Get the content that has the same ID as the data-toggle value
    var content = document.querySelector(toggleId);

    console.log(content);

    // ...

}, false);

That returned null.

I looked back at the logged toggleId value again, and realized it was set to the wrong ID. Our content area had an ID of #example, and the toggleId was set to #close. It was targeting an element that doesn’t exist.

I changed it to #example and the script started working as expected.

Debugging isn’t glamorous #

Debugging is not the most glamour part of writing code. It involves a lot of staring at code trying to figure out what’s wrong.

But it’s also an essential skill to have a developer.

If you struggle with debugging, I hope this mini series gave you an approach you can use to start debugging your own code more effectively.


🚀 Make 2018 the year you master JavaScript! My pocket guides and mini courses are short, focused, and made for beginners. You can do this!

Have any questions or comments about this post? Email me at chris@gomakethings.com or contact me on Twitter at @ChrisFerdinandi.

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