JavaScript: The Definitive Guide, Fourth Edition

try/catch/finally

The try/catch/finally statement is JavaScript’s exception-handling mechanism. The try clause of this statement simply defines the block of code whose exceptions are to be handled. The try block is followed by a catch clause, which is a block of statements that are invoked when an exception occurs anywhere within the try block. The catch clause is followed by a finally block containing cleanup code that is guaranteed to be executed, regardless of what happens in the try block. Both the catch and finally blocks are optional, but a try block must be accompanied by at least one of these blocks. The try, catch, and finally blocks all begin and end with curly braces. These are a required part of the syntax and cannot be omitted, even if the clause contains only a single statement. Like the throw statement, the try/catch/finally statement is standardized by ECMAScript v3 and implemented in JavaScript 1.4.

The following code illustrates the syntax and purpose of the try/catch/finally statement. In particular, note that the catch keyword is followed by an identifier in parentheses. This identifier is like a function argument. It names a local variable that exists only within the body of the catch block. JavaScript assigns whatever exception object or value was thrown to this variable:

try { // Normally, this code runs from the top of the block to the bottom // without problems. But it can sometimes throw an exception, // either directly, with a throw statement, or ...