Debugging Errors 




  • Read the error
    • including stack traces
  • Read the code
  • Add print statements (console.log in JavaScript)
  • Use a rubber duck or a pair partner
  • Use an Interactive debuggers

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:

  • 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
  • Remember to breathe!


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:

  • Where in your code is the error occurring?
  • What is the name of the error message?
  • Why is this error happening now?
    • What is the context of the error?
    • What is the code expecting, and what did it find instead?
  • If there are several error messages in a row, which is the root error?

(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?

  • There will often (though not always!) be a stack trace including paths, file names, line numbers... even column numbers!
  • A stack trace is basically a list of functions that have called each other, ordered from most to least recent
  • Once you mentally redact the stack trace, it becomes easier to understand the rest of the error.

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?

  • this will usually be in English, though possibly fragmented and jargony, e.g. "file not found" or "undefined is not a function".
  • This is not gibberish. It is meaningful. Names have power.


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

  • e.g. if it says "file not found", what is the file it is looking for? Is it looking in the right place and the file isn't there? Or is it looking in the wrong place?
  • e.g. if it says "undefined is not a function", undefined is the value of a variable that it expects to be a function. What is the name of the variable? Why doesn't it point to a function?

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?

  • errors often cascade, with one causing another causing another
  • remember to scroll up in your terminal to see the earlier error messages. Don't allow later errors to hide earlier ones.

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

  • JS console
  • inspect & edit DOM / CSS
  • network access logs and graphs

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:

  • set a breakpoint to pause your program when it reaches a specific line
  • inspect values of variables and constants
  • a code console which lets you execute expressions, call functions, or even change existing code during a pause
  • set watch expressions which will enable breakpoints based on conditions (like x < 2)

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