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JavaScript Closures

JavaScript variables can belong to the local or global scope.

Global variables can be made local (private) with closures.

Global Variables

A function can access all variables defined inside the function, like this:

But a function can also access variables defined outside the function, like this:

In the last example, a is a global variable.

In a web page, global variables belong to the window object.

Global variables can be used (and changed) by all scripts in the page (and in the window).

In the first example, a is a local variable.

A local variable can only be used inside the function where it is defined. It is hidden from other functions and other scripting code.

Global and local variables with the same name are different variables. Modifying one, does not modify the other.

Variable Lifetime

Global variables live until the page is discarded, like when you navigate to another page or close the window.

Local variables have short lives. They are created when the function is invoked, and deleted when the function is finished.

A Counter Dilemma

Suppose you want to use a variable for counting something, and you want this counter to be available to all functions.

You could use a global variable, and a function to increase the counter:

There is a problem with the solution above: Any code on the page can change the counter, without calling add().

The counter should be local to the add() function, to prevent other code from changing it:

It did not work because we display the global counter instead of the local counter.

We can remove the global counter and access the local counter by letting the function return it:

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