hide - Understanding jQuery

Scope
In order to understand function closures, one of the next subjects in the book, you must first
have an understanding of the concept of scope. Scope is where things are happening in your
program. Scope is usually determined by the { and } brackets. For example, the function “add”
we discussed earlier uses these brackets to define its scope. In other words, when the function
is executed, what is executed is the code within that scope.
We can call the function twice or three times, and again, over and over, the code contained
in the scope of that function will be executed. When a function is executed, a special part of
memory is allocated into which the function is loaded. Then this code is executed.
Unfortunately, the low-level details of computer program execution flow is not a part of this
book, and you don’t really need to know them to be a JavaScript or jQuery programmer. But
it’s an interesting subject you may want to explore on your own.
The most important concept you do have to understand with regard to scope, is that variables
in a scope are not visible to other scopes in a program.
What does not visible mean?
For example, variables defined inside a function scope only work within that function’s scope.
You can’t work with them from within other functions. That’s why variables can be passed to
functions as parameters, instead.
Most of the time, you will not be able to recall variables across scopes. In other words, the
scope the variable was created in, is the only scope it can be used in. But there is an exception,
and that is when we use a function closure, a concept we will define in a few moments.
Remember that all variables in the global scope are visible from everywhere. The global scope
is not limited to any brackets, it exists at the root of the program, “above” all other scopes.
Global Scope
Global scope is the root of your program. What you define in the global scope, things like
functions, variables, arrays, and so forth... will be visible from anywhere inside your program,
including, within all defined functions.
Also, on the contrary, variables defined inside a function cannot be visible in the global scope,
they are only visible within that function. Use scope to your advantage to organize data of your
program. Many programmers don’t even know that the concept of scope exists, they just follow
their instinct. There is nothing wrong with that. But being aware of it can be important if
you are looking to improve your understanding of writing code efficiently.
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