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Errors versus Exceptions

some other reason, perhaps, that the array has no items in it? There is no way to know why the invalid

value was returned. Errors and error handling help eliminate this conundrum.

With the incorporation of error handling into JavaScript, Web developers now have the ability to better

control code. Good error-handling techniques allow smooth developing, debugging, and deploying of

scripts. In many ways, such techniques are actually more important in JavaScript than other program-

ming languages because it lacks a standard development environment, such as Visual Studio.NET for

the .NET Framework or NetBeans for Java development, to guide developers.

Errors versus Exceptions

When talking about errors in programming, you really have only two categories of primary concern:

syntax errors and runtime errors.

Syntax errors, also called parsing errors, occur at compile time for traditional programming languages

and at interpret time for JavaScript. These errors are a direct result of an unexpected character in the

code and thus prevent it from being fully compiled/interpreted. For example, the following line causes

a syntax error because it is missing a closing parenthesis:

document.write(“test”;

When a syntax error occurs, the code cannot be executed. In JavaScript, only the code contained within

the same thread as the syntax error is affected. Code in other threads and other externally referenced

files still execute appropriately assuming nothing in them depends on the code containing the error.

Runtime errors, also called exceptions, occur during execution (after compilation/interpretation). In this

case, the problems are not with the syntax of the code. Rather, an operation attempting to complete is

illegal in some way. Example:

window.openMySpecialWindow();

In the previous code, an attempt is made to access a method of the

window

object named

openMySpecialWindow

. Syntactically this line is correct, however, no such method exists. This

causes the browser to return an exception.

Exceptions only affect the thread in which they occur, allowing other JavaScript threads to continue nor-

mal execution. Consider the following HTML page:

<html>

<head>

<title>Exception Test</title>

function handleLoad() {

window.openMySpecialWindow();

alert(“Loaded”);

}

function handleClick() {

alert(“Clicked”);

}

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