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Debugging Techniques

Note that the check for the

instanceof Error

must be the last condition in the

statement because

all the other error classes inherit from it (so a

SyntaxError

returns

true

when testing

instanceof

Error

Debugging Techniques

Before JavaScript debuggers were readily available, developers had to use creative methods to debug

their code. This led to the placement of strategically placed alerts, using LiveConnect to access the Java

console, using the JavaScript console, and throwing custom errors. Each of these techniques has its

advantages and disadvantages. Which one should you use? This section explains which debugging tech-

nique is right for you.

Using alerts

The most popular (although the most unwieldy) method of debugging is the placement of alerts strategi-

cally throughout code. For example:

function test_function() {

alert(“Entering function.”);

var iNumber1 = 5;

var iNumber2 = 10;

alert(“Before calculation.”);

var iResult = iNumber1 + iNumber2;

alert(“After calculation.”);

alert(“Leaving function.”);

}

JavaScript developers around the world turn to alerts as a quick and dirty approach for debugging. The

most popular way is to show alerts with descriptive text, as in the previous example. Some also use a

numbering method, starting the first alert at 0, the second at 1, and so on, to see where the code breaks.

This approach requires a lot of cleanup because you must remove the extra alerts when your debugging

is complete. Another problem occurs when dealing with infinite loops: If your script is causing an infi-

nite or long-running loop, alerts could keep popping up and prevent you from closing the browser. For

this reason, using alerts for debugging is best kept to small code segments.

All browsers report a developer-thrown error, but the way the error message is dis-

played may differ slightly. Internet Explorer 6 only displays the error messages

when throwing the

Error

object; otherwise, it simply says

Exception thrown and

not caught

without giving any of the details specified; Mozilla, however, reports

Error: uncaught exception:

and then calls the

toString()

method of the object

that was thrown.

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