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JavaScript: Test whether JavaScript variable is defined

Tip by Axuter | 2012-06-01 at 18:22

Sometimes we want to know whether a variable in JavaScript has already been defined before.

This is very easy using the following code:

if (typeof a == 'undefined') {
   alert('Variable "a" is not defined!');
} else {
   alert('Variable "a" is defined!');

In the example, we test whether the variable named "a" has been defined or not. For this we simply use the function "typeof", which returns the type of the variable. If the variable is not defined, "typeof" returns "undefined" and we know, that we do not have the variable defined before.


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