Understanding Error Messages

The first thing a beginner does when they see an error message is panic and close the terminal. But an error message is not a failure—it’s a clue.

Learning to read and understand error messages is the single most important part of becoming a good debugger.

How to Read an Error Message

An error message usually tells you three things:

1. What happened (The type of error).
2. Where it happened (The file and line number).
3. Why it happened (Additional context).

Real-World Example: JavaScript Error

Look at this error message:

ReferenceError: userName is not defined
    at loginUser (auth.js:15:5)
    at main (app.js:22:1)

Breakdown:

• ReferenceError: The type of error (You’re using a variable that doesn’t exist).
• userName is not defined: What exactly went wrong.
• auth.js:15:5: Where it happened (File auth.js, line 15, character 5).

Common Error Types (Across Languages)

You’ll see these error types over and over:

1. TypeError (or NullPointerException)

You tried to do something with a variable that isn’t the right type. Example: user.name.toUpperCase() where user.name is actually null.

2. SyntaxError

You wrote code that doesn’t follow the rules of the language. Example: if (true) { console.log("Hello" (missing closing brace).

3. ReferenceError

You tried to use a variable or function that hasn’t been created yet.

How to Investigate an Error

When you see an error message, follow these steps:

1. Read the whole message: Don’t just look at the first line.
2. Identify the “What”: What is the name of the error? (e.g., TypeError, AttributeError).
3. Identify the “Where”: Look for the file name and line number in your code (not in library code).
4. Google it! If you don’t understand the error, copy and paste the first line into Google. Thousands of other developers have probably seen it before!

Common Beginner Mistakes

• Not Reading the Message: Just assuming “it’s broken” without actually reading what the computer is telling you.
• Assuming the Error is in a Library: If you see an error in node_modules or a library, it’s almost always because you passed the wrong data into that library.
• Fixing the Wrong Line: Sometimes an error on line 50 is caused by a mistake on line 45. Check the lines around the reported error!

Advice for Beginners

• Error messages are your friends. They are trying to help you.
• Copy and Paste: When asking for help, always copy and paste the full error message, not just a screenshot of part of it.
• Look for the “Caused by” line: Some languages (like Java) show a long list of errors. Look for the “Caused by” at the bottom to find the real problem.

Next Lesson

Now that we can read the first line of an error, let’s learn how to trace the whole history of the crash:

➡ Reading Stack Traces