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Nested try...catch statements

The code in the

finally

clause behaves the same way as the

finally

clause in Java, containing code

that should be executed whether or not an exception occurs. This is useful for closing open connections

and freeing up resources. For instance:

connection.open();

try {

connection.send(data);

} catch (exception) {

alert(“An exception occurred.”);

} finally {

connection.close();

}

Nested try...catch statements

It is possible for an error to occur inside the

catch

clause of a

try...catch

statement. In this case,

using nested

try...catch

statements is the answer. Consider the following example

try {

eval(“a ++ b”); //causes error

} catch (oException) {

alert(“An exception occurred. “);

try {

var aErrors = new Array(10000000000000000000000); //causes error

aErrors.push(exception);

} catch (oException2) {

alert(“Another exception occurred.”);

}

} finally {

alert(“All done.”);

}

In this example, an error is thrown immediately and the first alert is displayed. When execution contin-

ues in the first

catch

clause, another error is thrown because of the attempt to create an array with too

many elements. Execution goes to the second

catch

clause and displays the second alert before continu-

ing on into the

finally

clause.

The Error object

So what exactly is it that the

catch

statement catches? Just as Java has a base class

Exception

to throw,

JavaScript has a base class called

Error

to throw. An

Error

object has the following properties:

name

— A string indicating the type of error

message

— The actual error message

Mozilla’s extensions to ECMAScript include the capability to add more than one

catch clause per

try...catch

statement. However, because this extension exists

only in Mozilla, it is not recommended.

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