[netstuff]

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Undefined data type
The undefined data type has one value, undefined, and is automatically assigned to a variable to which a value hasn’t been assigned, either by your code or user interaction.

The value undefined is automatically assigned; unlike null, you don’t assign undefined to a variable or property. You use the undefined data type to check if a variable is set or defined. This data type lets you write code that executes only when the application is running, as shown in the following example:

if (init == undefined) {
trace("initializing app");
init = true;
}

If your application has multiple frames, the code does not execute a second time because the init variable is no longer undefined.

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