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message property

The name of the

Error

object corresponds to its class (because

Error

is just a base class), which is one of

the following:

Class

Occurs When

EvalError

An error occurs in the

eval()

function.

RangeError

A number value is greater than or less than the numbers that can be repre-

sented in JavaScript (

Number.MAX_VALUE

and

Number.MIN_VALUE

ReferenceError

An illegal reference is used.

SyntaxError

A syntax error occurs inside of an

eval()

function call. All other syntax

errors are reported by the browser and cannot be handled with a

try...catch

statement.

TypeError

A variable’s type is unexpected.

URIError

An error occurs in the

encodeURI()

or the

decodeURI()

function.

The

message

property of an

Error

object is the browser-generated error message indicating the nature

of the error. Because this property is browser-specific, the same error can generate a different error mes-

sage in different browsers. Consider the following line of code:

eval(“a ++ b”);

This line alone causes a

SyntaxError

to be thrown because the ++ symbol isn’t valid in this context. The

error message from Internet Explorer 6 is

“Expected ‘;’”

whereas Mozilla 1.5 provides

“missing ;

before statement.”

The

message

property can be used to display a more meaningful message to users while preventing the

browser from reporting the error directly:

try {

window.nonExistentFunction();

alert(“Method completed.”);

} catch (oException) {

alert(“An exception occurred: “ + oException.message);

} finally {

alert(“End of try...catch test.”);

}

Both Mozilla and Internet Explorer have extended the

Error

object to suit their own

needs. Mozilla provides a

fileName

property to indicate which file the error occurred

in, a

lineNumber

property indicating the line that the error occurred on, and a stack

property containing the call stack up to the point of the error; Internet Explorer pro-

vides a

number

property to indicate the error number.

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Error Handling

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