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Determining the type of error

Despite being limited to only one

catch

clause per

try...catch

statement, you have a couple of easy

ways to determine the type of error that was thrown. The first is to use the

name

property of the

Error

object:

try {

eval(“a ++ b”); //causes SyntaxError

} catch (oException) {

if (oException.name == “SyntaxError”) {

alert(“Syntax Error: “ + oException.message);

} else {

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

}

The second way is to use the

instanceof

operator and use the class name of different errors:

try {

eval(“a ++ b”); //causes SyntaxError

} catch (oException) {

if (oException instanceof SyntaxError) {

alert(“Syntax Error: “ + oException.message);

} else {

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

}

Raising exceptions

The third edition of ECMAScript also introduced the

throw

statement to raise exceptions purposely. The

syntax is the following:

throw

error_object

;

The

error_object

can be a string, a number, a Boolean value, or an actual object. All the following

lines are valid:

throw “An error occurred.”;

throw 50067;

throw true;

throw new Object();

It is also possible to throw an actual

Error

object. The constructor for the

Error

object takes only one

parameter, the error message, making it possible to do the following:

throw new Error(“You tried to do something bad.”);

All the other classes of

Error

are also available to developers:

throw new SyntaxError(“I don’t like your syntax.”);

throw new TypeError(“What type of variable do you take me for?”);

throw new RangeError(“Sorry, you just don’t have the range.”);

throw new EvalError(“That doesn’t evaluate.”);

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