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 ...