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Part 2: Debugging Logic Errors

Logic errors are more difficult to debug than syntax errors because they're not something that outwardly looks wrong. The syntax of the code is written correctly, but it's just not doing what you want it to do. Debugging logic errors requires more work, but the JavaScript Debugger tool makes the work much easier than it could be.

Before you begin debugging, let's take a look at the problem. With the contact form loaded in Dreamweaver, press the F12 key to preview the page. The JavaScript code is written so that when you click the Submit button, it checks whether each field has been filled in.

Type something into only the Comments text box, and then click the Submit button. Notice how the JavaScript code correctly displays an error message saying that all of the fields, except for Comments, need data (shown in Figure 5).

Figure 5Figure 5 Logic working correctly.

To see where the logic error occurs, leave all the form fields blank and then click the Submit button. Notice how the error message displays only -- Your comments, as shown in Figure 6. Because all the fields are blank, it should display all the fields names, but for some reason it doesn't. There's the logic error that needs to be fixed.

Figure 6Figure 6 The logic is not working correctly—other blank fields should be listed.

Setting Breakpoints

To begin debugging the logic error, you'll need to set a breakpoint. Breakpoints are "flags" that you set on a specific line of code, to tell the JavaScript Debugger to pause the execution of the code before that line runs. When the code is paused, you can check the values of variables and step through the code one line at a time.

To set a breakpoint, in Dreamweaver's Code view, click on line 51: if (comments == "") {.

Next, click on the Code Navigation icon and select Set Breakpoint, as shown in Figure 7.

Figure 7Figure 7 Setting a breakpoint.

A red dot appears to the left of line 51, indicating that a breakpoint for that line has been set (see Figure 8).

Figure 8Figure 8 Confirming that a breakpoint was set.

Why Is the Breakpoint on Line 51?

At this point, you may be wondering why line 51 was chosen for a breakpoint. Line 51 was chosen based on a hunch; during our testing, that's where the validation logic begins for the Comments text box—and where things seem to go wacky. The Comments box seems to be causing the problem. In the next step, we'll run the Debugger and see if that breakpoint helps us confirm the source of the logic error.

Launch the Debugger again by clicking Preview/Debug in Browser and then clicking Debug In iexplore (or whatever your browser name is), as shown in Figure 7. A number of windows open. Find the Security Warning window, shown in Figure 9, and click Yes. This window authorizes Macromedia to run the JavaScript Debugger application and control the Web browser window launched for debugging.

Figure 9Figure 9 Security Warning window.


Find the Web browser window that launched when you ran the JavaScript Debugger. The Web browser's window will be blank; a prompt appears asking if you want to start debugging. Click OK.

Next, find the JavaScript Debugger window and click the Run (F8) icon, located in the top-left corner, as shown in Figure 10. This tells the browser to run all of the code (HTML, JavaScript, etc.) until the breakpoint is reached.

Figure 10Figure 10 Clicking the Run (F8) icon runs all the code up to the first breakpoint.

The JavaScript Debugger window (see Figure 10) is now controlling the Web browser window with the "contact us" form. You can tell which Web browser window is controlled by the JavaScript Debugger because it will say <debugger> in the title bar, as shown in Figure 11.

Figure 11Figure 11 Identifying the browser used for debugging.

Now it's time to begin stepping through the code to determine where the logic error occurs. In the browser window that the "contact us" form loaded, click the Submit button at the bottom of the form (leave all fields blank).

The JavaScript Debugger window (see Figure 12) detects that you clicked the Submit button, runs the JavaScript code, and stops at the breakpoint you set on line 51. Notice that on line 51, there's a small yellow arrow inside the red circle, which represents the breakpoint. This arrow signifies that the JavaScript form validation code has executed up to that line but has not executed that line.

Figure 12Figure 12 The JavaScript Debugger window.

Watching Variables

With the JavaScript Debugger pausing the Web browser's code at the breakpoint, you can now begin to "watch" your variables. When you watch a variable, you're monitoring how the variable's value changes (in "real time") as you step through the code.

The variable that holds the error message for your "contact us" form is errormsg. To watch the variable errormsg, follow these steps:

  1. On the middle/left side of the Debugger window, click the New Item (+) icon, as shown in Figure 12.

  2. In the Variable Name column, the string <variable name> appears. Replace it with errormsg and then press the Tab key, or click somewhere else in the Debugger.

The column to the right of Variable Name is the Value column. The Value column displays the value of errormsg, as shown in Figure 13.

Figure 13Figure 13 The lower half of the JavaScript Debugger watching a variable.

To quickly review, the JavaScript code checks each field to see whether has been filled in. If it has not, it appends the name of that field to the variable errormsg. So far, watching the errormsg variable has indicated that the code is working normally. You submitted the form without filling in any fields, and the error message contains error text for each of those fields (so far).

Next, let's step through the validation of the Comments text box and see what happens.

Stepping Through Code

The JavaScript Debugger has several options for stepping through code:

  • Step Over (F9)—Step Over allows you to execute one line of code at a time. When you're on a line that executes an if statement and you press Step Over, it checks to see whether the condition in the if statement is met. If the condition is met, the debugger moves to the next line. When the condition in the if statement is not met, the debugger jumps to the else portion of the statement (assuming that there is an else portion) or jumps to the next section of code.

  • Step Into (F10)—Step Into allows you to step into a function and monitor the execution of each line in that function. Once inside the function, you can use Step Over to move line by line.

  • Step Out (F11)—Use Step Out when you have stepped into a function, have verified what you wanted to see, and want to automatically execute the rest of the function without having to step over each remaining line of the function. Once the Debugger completes running through the function, you can continue stepping through the rest of the code line by line (if you want).

To execute the code on line 51, click the Step Over Code icon, at the top of the JavaScript Debugger, as shown in Figure 14.

Figure 14Figure 14 Stepping over code.

The code executed and the yellow arrow on the left side of the Code view is now on line 52. This tells you that the code correctly determined that the Comments box was not filled in and is now ready to run the code on line 52.

Click the Step Over icon again. Aha! When the code on line 52 was executed, it erased the contents of errormsg and inserted -- Your comments \n instead of appending the text. This happened because you used the = assignment operator when you needed to use the += assignment operator. = means "replace the value of a variable with this new value." += means "append this value to the current value of the variable."

Now that you have isolated the problem, you can fix the code. Close the JavaScript Debugger by clicking the x in the top/right corner. Also close the Debugger Web browser window with the "contact us" form.

Go back to Dreamweaver's Code view and add a + sign in front of the = on line 52 so that it reads like this:

  52  errormsg += " -- Your comments \n";

Save your page and then test it by pressing F12 to preview it in a browser. When the page loads, leave all fields blank and then press the Submit button. Your error message should include all the blank fields, as shown in Figure 15.

Figure 15Figure 15 The correct error message after fixing the code.

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