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! But 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:


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?

Debugging with Unit Tests

A unit test is a little side program that executes your main program and makes sure its behavior is correct.

When a user reports a bug -- or when a coder notices one -- the first thing to do is to write a test that exposes the bug, then run it and watch it fail.

This assures that you have actually identified the cause of the bug, and that once you think it's fixed, it's actually fixed.

Rubber Duck Debugging

Often, simply explaining your code reveals its flaws.

This is one reason why pair programming is so effective.

If you don't have a pair, try explaining your problem out loud to a pet, or a doll.

rubber duck

Read the Error

Step 1: Read the error... carefully.

Mentally dissect the error report. Try to separate the signal from the noise.

Answer the following questions:

(More detail on these questions in the following slides.)

After answering these questions, look at your code. Open up the file, scroll to the line and read the code to yourself, or read it aloud to your pair partner or your rubber duck. Many times the cause will be obvious.


Where in your code is the error occurring?

Read the first section of this article, JavaScript Errors and Stack Traces in Depth: How The Call Stack Works , to learn more about Stack Traces in JavaScript


What is the name of the error message?


What is the context of the error? What is the code expecting, and what did it find instead?

You may have to use traditional debugging techniques to answer these questions, e.g. print statements to fill in the values that didn't make it into the error message itself.

(Yes, a 'file not found' error really should tell you what file was not found; when you write your own error messages you should include as much relevant info as you can.)


If there are several error messages in a row, which is the root cause?

In-Browser Debugging

Modern Web Browsers (Chrome, Firefox, Safari, Edge) have some very good tools to help you inspect the state of your app, including

These tools are useful, but you still need to ask and answer all the debugging questions yourself.

"Modern Front-End Workflow with DevTools" talk:

Interactive Debuggers

Many text editors contain an interactive debugger (aka visual debugger) with some powerful features, including:

These features can help you understand what your code is doing in why, but can also be tedious or complicated or distracting; often print statements and unit tests are a more efficient path to identifying and fixing bugs.

Visual Studio Code has a great interactive debugger -- watch this video overview to learn more

LAB: exercises about debugging

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