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APPENDIX

Prior to running this example, if we were unfamiliar with closures, we might

look at this code and see some problems. We might surmise that, because the call-
back will fire off three seconds after the page is loaded (long after the ready han-
dler has finished executing), the value of

local

is undefined during the execution

of the callback function. After all, the block in which

local

is declared goes out of

scope when the ready handler finishes, right?

But on loading the page and letting it run for a short time, we see the display

as shown in figure A.5.

It works! But how?
Although it is true that the block in which

local

is declared goes out of scope

when the ready handler exits, the closure created by the declaration of the func-
tion, which includes

local

, stays in scope for the lifetime of the function.

NOTE

You might have noted that the closure, as with all closures in JavaScript,
was created implicitly without the need for explicit syntax as is required in
some other languages that support closures. This is a double-edged sword
that makes it easy to create closures (whether you intend to or not!) but
can make them difficult to spot in the code.

Unintended closures can have unintended consequences. For exam-

ple, circular references can lead to memory leaks. A classic example of
this is the creation of DOM elements that refer back to closure variables,
preventing those variables from being reclaimed.

Figure A.5 Closures allow callbacks to access their environment even if that
environment has gone out of scope.