• Read the error

    • including stack traces
  • Read the code

  • Add print statements (console.log in JavaScript)

  • Use an Interactive debugger

    • in the browser

    *in VSCode

  • Use a rubber duck

What is Debugging?

Debugging is fixing code that you thought was working... but isn't working.

It's solving a puzzle that you created for yourself.

This can be frustrating! It can also be rewarding, and can lead to better understanding of your program and of the context in which it operates.

Errors vs Failures

Roughly speaking, there are two scenarios for debugging:

  1. failures: the program runs to completion, but gives an incorrect or unexpected result
  2. errors: the program exits with an error message

Debugging Failures

Debugging is an art, not a science. But its principles are scientific:

  • Make a mental model of the system

  • Think of hypotheses

  • Make observations to confirm or reject those hypotheses

  • Focus on anomalies

  • Change one thing at a time and observe the results

Debugging with Print Statements

The most fundamental way to debug is to print the values of variables at various points in your code.

function factorial(x) {
    console.log("in factorial: x=" + x);
    if (x < 1) {
        return x;
    } else {
        console.log("multiplying " + x + " by factorial(" + (x-1) + ")")
        return x * factorial(x - 1);

Keep adding print statements until you find an anomaly; repeat until you find the source.

Don't forget to remove the print statements after you've fixed the bug!

Debugging with Print Statements

Output of the above:

> factorial(4)
in factorial: x=4
multiplying 4 by factorial(3)
in factorial: x=3
multiplying 3 by factorial(2)
in factorial: x=2
multiplying 2 by factorial(1)
in factorial: x=1
multiplying 1 by factorial(0)
in factorial: x=0

Can you figure out the bug?


A 'breakpoint' is a location in your code where you tell the program "Stop! And wait for me to tell you to continue." Break points can are an integral part of any debugging tool and allow you to step through your code line by line (or chunk to chunk) to figure out exactly where things went wrong

Browser Tools

If you're running a program through your browser most modern browsers (i.e. Chrome, or FireFox) have built in debugging tools you can use!

Step 1: Accessing the Dev Tools

In Chrome:

  • ctrl + shift + i for a Windows machine

    • or right click and select inspect from the drop-down menu
  • cmd + option + i for a Mac

    • or option + click and select inspect from the drop-down menu

In FireFox:

  • ctrl + shift + c for a Windows machine

    • or right click and select Inspect Element from the drop-down menu
  • cmd + option + c for a Mac

    • or option + click and select Inspect Element from the drop-down menu

Step 2a: Add Breakpoints in Chrome

  • In the dev tools, navigate to the Sources tab (to the immediate right of the console tab)

  • Select the file you want to debug

  • Click on the line number where you want to add your breakpoint

Note: If you're unsure where your error is coming from you can add abreak point at the top of your code and step through your whole program line by line

Step 2a breakdown

chrome debugger example

Step 2b: Add Breakpoints in FireFox

  • In the dev tools, navigate to the Debugger tab (to the immediate right of the console tab)

  • Select the file you want to debug from the Sources section

  • Click on the line number where you want to add your breakpoint

Step 2b breakdown

firefox debugger example

Step 3: Run your Code

Once you've added a breakpoint run your code, and step through the section by clicking the step over button or pressing f10.

If you've stepped through your code, and are still having issues finding where the errors are coming, or you find yourself traversing through previously unknown code there are several things you may want to try:

  • Disable extensions

  • Run in 'incognito' mode to make sure you're not caching bad code

  • Try running the code in a different browser

  • Try running it in a 'pristine' browser (a fresh install with no extensions or settings changes)

Note: If your code runs on page load, you will need to refresh the page before the debugger takes effect.

VSCode's Debugger

There is also a debugger tool built right into VSCode. This debugger allows you to set breakpoints by clicking just to the left of the line number. A faded red circle will appear when you hover over a valid location, and a solid red circle will appear at the breakpoint once you click.

If you click the Debug icon on the left side of your editor it will open a box to the left that will display outputs.

VSCode Debugger Example

vscode debugger

But Wait! There's More!

The VSCode debugger also allows you to track the value of variables or expressions through the execution of your code. To add a variable to watch:

  • go to the WATCH section of the box(?) to the left

  • Click the + (Add Expression) button and type in the variable name you want to track

  • Step through your code and watch as the variable changes (or doesn't)

  • Look at the CALL STACK section (just below WATCH).

    • Where did your code break?
    • Was your tracked value what you expected it to be at this point?

Rubber Ducky Debugging

Rubber Ducky debugging is a simple way to debug a problem with no technology required.

  • grab an inanimate object, preferably made of rubber and shaped like a water fowl

  • Tell it what you're trying to do

  • Explain what you think your code is doing

Often times talking through a problem will help you find a solution.

You can also do this with other people! You don't have to use an inanimate object.