The undefined type is generally assigned to those variables that are declared but not yet initialized (i.e. var text;).

Keep in mind that if a variable is not declared, the script will stop causing an error.

Undeclared identifiers are not the same as undefined ones.

Following examples shows the difference between them:

var text; // undefined

// var zipcode; //left out due comments

alert (test); //undefined

alert (zipcode); // cause an error!

However the operator typeof may be applied to undeclared identifiers and it will, confusingly, return undefined as the type.


The undefined data type example:


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