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Error Handling

Traditionally, JavaScript has been known as a language filled with confusing error messages such

Object Expected

and

Illegal Syntax

with nothing more than a line number to identify its origin.

Debugging such messages was a painful experience at best, which is why the version 4 browsers

(Internet Explorer 4.0 and Netscape Navigator 4.0) included some basic error-handling functional-

ity. Shortly thereafter, an answer came from the ECMA in the form of ECMAScript, third edition.

This latest edition of ECMAScript added exception handling capabilities modeled after its big

brother, Java. Using some of the reserved words from the second edition of ECMAScript, the third

edition implements the

try...catch...finally

construct as well as the

throw

operator.

This chapter explores both the browser-based error-handling capabilities as well as ECMAScript’s

exception-handling features.

The Importance of Error Handling

In early browsers (such as Internet Explorer 3.0 and Netscape Navigator 3.0), there was no error

handling. Typically, functions return an

invalid

value (often

null

false

, or –1, depending on the

use case) to indicate that an error has occurred. Consider the following code:

var iLoc = findItem(colorarray, “blue”);

if (iLoc == -1) {

alert(“The item doesn’t exist. “);

} else {

alert(“The item is in location “ + iLoc + “.”);

}

In this case, the function

findItem()

returns a –1 (an invalid value) when the given string doesn’t

exist. Presumably, a valid value is a number greater than or equal to 0. But why did the function

return the invalid value? Does the string

“blue”

simply not exist in

colorarray

? Or is there

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