17: Ten (Or So) Tips for Debugging Your Scripts in Java

Drawer Quick Response Code in Java 17: Ten (Or So) Tips for Debugging Your Scripts

Getting into the habit of commenting on your JavaScript code as you write it can be a great help when it comes time to debug that code (You might be surprised at how much you can forget between the time you create a script and the time when your code misbehaves, which can be weeks or even months down the line!) If you create pseudocode to help you plan and design your scripts, try using that pseudocode as the basis for your JavaScript comments Doing so helps the future you (or someone else who has to debug your script) understand precisely what the code is trying to accomplish

If you encounter a genuine bug, you need to try to home in on it and identify precisely which lines of code are affected Here are some examples to help you work backward from the clues: Does the problem occur the instant the page loads If so, the problem is probably either HTML-related or in the JavaScript code you set up to handle the onLoad event Does the problem occur when users type text in an input field Check the onChange and onBlur event handlers associated with that field When users click a button, do things go haywire Check that button s onClick event handling code Does something go wrong when users close the window The culprit is probably lurking in your onUnload event-handling statements First, decide on a place to begin your search say, with the function that s called from one of your onClick event handlers (I call it buggyFunction()) The next step is to dig a little deeper For example, try adding a test button to your JavaScript code that exercises that same function as the following one does:

In the first line, you re sending the buggy function numeric and string literals This process helps you determine whether the function itself is buggy or whether the problem lies with the variables that your original code is passing to the function

If the function behaves incorrectly after you pass it numeric and string literals, you know that the bug is in your function If it behaves correctly, you need to check the parameters that the original onClick is sending to the function (See the Displaying Variable Values section later in this chapter) One way to isolate a bug is to comment out all the code in a function except one or two suspect statements By using this approach, you can focus on the statements that you want to examine in more detail To comment out a section of code, you place JavaScript comments before each line, like so:

// someVariable = someResult;

Make a copy of your original HTML or script file before you make any changes Few things are more frustrating than modifying a file beyond recognition, only to have it perform even worse than when you started and then forgetting how the code originally looked! You might also want to look into a version control tool such as CVS (wwwcvshomeorg) Version control tools allow you to track different versions of files separately so that if you accidentally goof up one version, you can always go back to an earlier, working version

The JavaScript Guide and JavaScript Reference are the most up-to-date resources available regarding the JavaScript language as Netscape implements it Bookmarking or downloading these documents in your browser helps ensure that they re at your fingertips when you need them!

Internet Explorer implements JavaScript through the Microsoft scripting language JScript Check out the following URL for a complete description of all things JScript-related:

A useful debugging technique involves displaying the values of variables at various stages in their lives For example, suppose that you have a function whose job is to calculate the total cost of an order Based on your understanding of the way the total should be calculated, you determine that this function always returns an incorrect value; you just don t know why