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Chrome Dev Tools and Debugging JS


After this lesson, you will be able to:

  • Identify and resolve common and uncommon "logical errors" that affect the results of your program.
  • Use logs to troubleshoot errors in an application.


Before this lesson, you should already be able to:

  • Create variables in JavaScript.
  • Differentiate between data types (strings, numbers, Booleans).
  • Use conditionals to control program flow based on Boolean conditions.
  • Create arrays and access/manipulate elements in arrays.


  1. Fork and clone this repository.
  2. Change into the new directory.
  3. Add a remote back to the original class repo (not your own forked version)..hint: git remote add remote class-repo-url

Tip Of The Day

Open files in Chrome using the terminal by copy/pasting the following into your terminal. It must be >> instead of >. Running this will add a shortcut to your .bash_profile to open files in Chrome

echo "alias chrome='open -a \"Google\"'" >> ~/.bash_profile


  • Review current lesson objectives.

Oftentimes, the toughest part of troubleshooting is identifying the exact cause of the problem.

In this course, we'll teach you to identify and decipher errors.

What does it mean to have an error in JS?

Errors don't just happen. Chrome doesn't just "break." In fact, it's not Chrome that's throwing the error at all — it's JavaScript.

Whenever you run into an error, it's because somewhere in the code you're running (or in JavaScript's source code) there's a line that says: "When this happens, throw an error."

What's an Error?

Let's see how to throw an error.

Here's a piece of HTML/JS that manually "throws" an error...

<!DOCTYPE html>
    <title>JS Errors</title>
    	throw(new Error("Oh, noes."));
    	console.log("Such is life.");

The result:

Uncaught Error: Oh, noes.       index.html:6

Note that the line with console.log("Such is life.") will never run. This is one of the things that can make writing JavaScript frustrating at first. One little error in your code can prevent any of your code from running!

Note: Don't worry too much about the syntax of how to throw an error; you won't be creating your own custom errors very often in this class. We're just showing this so you know what "magic" is going on under the surface.

Try Catch Finally Blocks

Good reference from W3Schools on try/catch/finally

<!DOCTYPE html>
    <title>JS Errors</title>
	    try {
	    	throw(new Error("Oh, noes."));
	    }catch(err) {
	       console.log("Such is life.", err);
	    }finally {
	       console.log("any last tasks to do?")

Independent Practice: Common Errors

Now that we know a little bit about what an error is and when it is thrown, let's look at some of the most common errors that you will run into in JS.

Encountering Common Errors in JS

For the following exercise:

  1. Open Chrome.
  2. Open Dev Tools (right click, "inspect").
  3. Open the console tab.

Take five minutes to independently work through this exercise.

You will first copy the code under each step and paste it into the console, and then hit "enter."

When you do, you should see an error message with a description of the error added to the console.

For each step, fix the error and then run the code in your console again until the code for that step is error-free.

Resist the urge to fix the error before running the code in your console if you spot it right away. The goal here is to see what errors you might encounter in a program.

var myString = "awesome";

function printString (str) {

var total = 0;

var cashRegister = {
  addToTotal: function (amount) {
    return total + amount;

total = cashRegister.add(4);
function shoutLongWords (word) {
  if (word.length > 5)
  } else {

Reading Error Messages

JavaScript has 7 error types. Three of them will account for 99% of the errors you encounter in this class, so we're going to focus on those.

An additioanl reference from W3Schools error types try/catch/finally

Check: What might these three words mean in the context of JavaScript?

  • Syntax
  • Reference
  • Type

Tools for Debugging

Note: Instructor to walk students through confirming that that the "Preserve log" checkbox is not checked in Chrome console. When it's checked, it doesn't clear error messages when you refresh your page. This can make your console get really ugly really quickly.

Reading Error Messages

Let's say I get this error message:

Uncaught SyntaxError: Unexpected token {      controller.js:8
Question: In what file and on what line is the error?

Click on controller.js:8 and it will show you that specific line of code.


More often than not, errors are usually caused by some variable not having the value you expect.

The easiest way to debug variables is to see how the value of a variable changes over time by placing console.log statements in our script.

From there, we open the console and look for the place where our variable stops having the value we expect it to have. That will help us pinpoint the error.

Be thoughtful about your debugging! Instead of just placing console.log statements everywhere in your code, first start by making small hypotheses about what could be wrong. Then use console.log statements to prove those hypotheses to be true or false, leading to intentional and specific changes in your code.

Let's see this in action by taking a quick look at a code example.

Independent Practice: Debug & Fix (20min)

Check out the starter code inside starter_code/practice_01 for a calculator program that isn't running correctly. Don't scan the JS file for errors. Open up your Dev Tools and follow their errors to figure out what's wrong.

Note: No need to clone, just follow along with the instructor on the board.

How To Find Answers

But let's say you've tried to solve the problem and think for yourself, and you're still stuck?

We're going to cover all the topics you need in this course, but we also want to provide you with important self-help resources. One of the best is surprisingly simple: Google.

Professional web developers often joke that a third of being a real programmer is just knowing how to find answers via Google.

This may be an exaggeration, but the point is not to worry if you ever find yourself scouring the web for the solution to a difficult coding problem. Even professionals do this, and it's totally normal.

Google Fu

When you search Google, it's important to include the programming language you're using in your search to narrow the results.

For example, if you're looking to add an alert message to your page using JavaScript, you should search "alert JS" or "alert JavaScript."

Only searching for "alert" will likely provide results that are far too broad.

As you know, Googling something doesn't always lead to a perfect answer or an exact solution.

Let's say we get the following error in the console...

Uncaught ReferenceError: robins_spatula is not defined

How should we go about Googling a solution to this error?

A Developer's Guide to Google Fu:
  1. Copy and paste the exact text of your error into Google, and then remove any words that are specific to your script.

    • For example, instead of:
    Uncaught ReferenceError: robins_spatula is not defined

    Uncaught ReferenceError: is not defined
  2. If you're looking for a specific phrase, put it in quotes.

    • is not defined will return any page with the words is, not, and defined.
    • "is not defined" will return any page with the exact phrase is not defined.
  3. Use - to exclude stuff.

    • ReferenceError -jquery will return any page with ReferenceError and without jquery.
  4. Use to search within a site

    • ReferenceError will search for pages with ReferenceError inside Stack Overflow only.

Stack Overflow is a Q&A forum where most of the questions you'll run into have probably already been asked and answered (we suggest you scroll down to find the answer with the most number of votes).

When the instructors look at things on Stack Overflow, we tend not to read the actual content of the question; we skip straight to the answers.

If the answer doesn't look promising, go to the next one. Repeat until the answers have very few upvotes.

If none of the answers are promising, go on to the next thing that turned up on Google.

You can get badges and special priveledges on Stack Overflow by asking good questions and giving good answers!

Mozilla Developer Network

Mozilla Developer Network (MDN) is another excellent resource. It contains loads of tutorials, references, and documentation for any questions you might have regarding HTML, CSS, and JavaScript.

Independent Practice (20min)

Check out the starter code inside starter_code/practice_02 for a fizzbuzz program that isn't working. Don't scan the JS file for errors. Open up your Dev Tools and follow their errors to figure out what's wrong.

One More Challenge (15min)

Open the file stacktrace.html file and see if you can figure out how to fix the one single error in the inline JavaScript. Open up your Dev Tools and view the error and figure out what's wrong.

Bonus Material

Instructor will review adding debugger keywork and breakpoints to troubleshoot errors.


  • Review independent practice solution

In this lesson, we've taken some steps toward being able to more efficiently solve problems and figure out how to find answers when we encounter errors in our code.

Debugging is a skill that takes time and practice to learn. We recommend you keep your console open at all times when working through JavaScript problems.

Developer tools are the first place developers look for clues when something is not working as expected. It takes much less time to quickly glance and see an error message pinpointing the issue and the file/line of code than to scan through all of your JavaScript and manually try to pick out the error.