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Understanding error messages

From: Foundations of Programming: Fundamentals

Video: Understanding error messages

Most programming languages will end up bombarding you with error messages when you're writing your code. This might be annoying, but you actually want this. You want it to tell you what's wrong. JavaScript is one of the few areas where we don't get automatically bombarded with error messages about syntax and that's because of the nature of where it's used. If you have errors in your JavaScript and I'm looking at script with exactly the same problem here, it's not showing up by default in the web browser, because you don't really want every user who is on your website seeing all the errors you might have accidentally put into your code.

Understanding error messages

Most programming languages will end up bombarding you with error messages when you're writing your code. This might be annoying, but you actually want this. You want it to tell you what's wrong. JavaScript is one of the few areas where we don't get automatically bombarded with error messages about syntax and that's because of the nature of where it's used. If you have errors in your JavaScript and I'm looking at script with exactly the same problem here, it's not showing up by default in the web browser, because you don't really want every user who is on your website seeing all the errors you might have accidentally put into your code.

So most browsers do realize when there are errors in JavaScript, but they hide them from you. We have to tell the browser we want to see the errors. Now, each web browser does this a bit differently, but what I'm about to show you does have its equivalent in Internet Explorer, Safari, Opera, Konqueror, all modern web browsers, but I'm going to show it in Firefox, which is my preferred browser for debugging JavaScript. So I'm on Firefox 6. I'm going to go up to the Tools menu, and find the Web Developer area, and find this thing called the Error Console.

Things do get rearranged, so you may find it under a slightly different menu but you're looking for the Error Console in Firebug. What will happen is it will pop up this separate window. I'm going to clear that and just keep that open in the background while I'm experimenting with the page. So it's not showing me anything right now, but as I move this page, I'm going to go around and say click on this. I click on Interesting Headline three times and over here in the Error Console I've got three errors. Now, it's actually telling me, the error is fourthfunction is not defined.

Meaning, you're trying to use something called fourthfunction, I don't know what that is. In fact, I even have a link to the file where the problem is and it's telling me the issues on line 9. If I click the link, it opens up a view of that and even highlights the line for me. So I can go straight to that line and figure out, okay, there is the problem, and hopefully I see that it's because I'm spelling fourthfunction with a lowercase f, whereas the actual definition of the function uses the uppercase F for the word Function here.

If you have other errors in syntax, you may find several of them showing up as soon as you load the page. General rule of thumb, if you've got multiple ones, go from the top down. Fix the first one first, because early errors can actually upset the rest of your code. Well, that's the Error Console in Firefox. There is also an option called the Web Console which is equivalent, but actually gives you this window at the top of the page and as I click in here, we get the same message. fourthfunction is not defined.

This also gives you extra information about your HTML and your CSS and so on. New in Firefox 6 is something that they've called the Scratchpad, which is actually an area we can just type or paste in some JavaScript, and then run it directly from this Execute menu. As of when I'm recording this, this is still early days, but it might be a nice environment just to play around with some sample code. These things are really useful, but they will only take us so far. For anything that's a bit more involved than simple syntax problems, we need a real debugger and with Firefox, we can get that too.

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This video is part of

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Foundations of Programming: Fundamentals

61 video lessons · 84510 viewers

Simon Allardice
Author

 
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  1. 4m 14s
    1. Welcome
      1m 16s
    2. Making the most of this course
      2m 8s
    3. Using the exercise files
      50s
  2. 22m 11s
    1. What is programming?
      5m 45s
    2. What is a programming language?
      4m 48s
    3. Writing source code
      5m 34s
    4. Compiled and interpreted languages
      6m 4s
  3. 16m 29s
    1. Why JavaScript?
      4m 45s
    2. Creating your first program in JavaScript
      6m 54s
    3. Requesting input
      4m 50s
  4. 31m 38s
    1. Introduction to variables and data types
      5m 16s
    2. Understanding strong, weak, and duck-typed languages
      3m 51s
    3. Working with numbers
      5m 4s
    4. Using characters and strings
      4m 5s
    5. Working with operators
      4m 47s
    6. Properly using white space
      6m 46s
    7. Adding comments to code for human understanding
      1m 49s
  5. 24m 49s
    1. Building with the if statement
      7m 35s
    2. Working with complex conditions
      4m 10s
    3. Setting comparison operators
      6m 59s
    4. Using the switch statement
      6m 5s
  6. 17m 56s
    1. Breaking your code apart
      4m 1s
    2. Creating and calling functions
      2m 57s
    3. Setting parameters and arguments
      6m 7s
    4. Understanding variable scope
      2m 23s
    5. Splitting code into different files
      2m 28s
  7. 13m 32s
    1. Introduction to iteration
      4m 28s
    2. Writing a while statement
      5m 24s
    3. Creating a for loop
      3m 40s
  8. 19m 28s
    1. Cleaning up with string concatenation
      4m 30s
    2. Finding patterns in strings
      8m 3s
    3. Introduction to regular expressions
      6m 55s
  9. 19m 59s
    1. Working with arrays
      5m 47s
    2. Array behavior
      5m 29s
    3. Iterating through collections
      5m 18s
    4. Collections in other languages
      3m 25s
  10. 10m 50s
    1. Programming style
      5m 55s
    2. Writing pseudocode
      4m 55s
  11. 25m 55s
    1. Input/output and persistence
      3m 6s
    2. Reading and writing from the DOM
      8m 11s
    3. Event driven programming
      7m 47s
    4. Introduction to file I/O
      6m 51s
  12. 24m 26s
    1. Introduction to debugging
      5m 57s
    2. Tracing through a section of code
      7m 5s
    3. Understanding error messages
      3m 21s
    4. Using debuggers
      8m 3s
  13. 14m 17s
    1. Introduction to object-oriented languages
      5m 18s
    2. Using classes and objects
      6m 29s
    3. Reviewing object-oriented languages
      2m 30s
  14. 11m 14s
    1. Memory management across languages
      5m 11s
    2. Introduction to algorithms
      4m 2s
    3. Introduction to multithreading
      2m 1s
  15. 29m 20s
    1. Introduction to languages
      1m 42s
    2. C-based languages
      4m 40s
    3. The Java world
      3m 13s
    4. .NET languages: C# and Visual Basic .NET
      6m 17s
    5. Ruby
      3m 4s
    6. Python
      2m 56s
    7. Objective-C
      4m 3s
    8. Libraries and frameworks
      3m 25s
  16. 1m 2s
    1. Where to go from here
      1m 2s

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