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Testing if a Javascript variable is defined

Posted October 23rd, 2007 in Javascript (Updated October 23rd, 2007)

Sometimes in Javascript you need to be able to determine if a variable has already been defined before continuing with your script. It is very simple to do this using the typof operator, as show in the example below:

var has_foo = typeof foo != 'undefined';
if(has_foo) {
    do_something();
}
else {
    do_something_else();
}

If the "has_foo" variable in the above example was not going to be used again, then the Javascript code could be made slightly smaller, and the logic reversed:

if(typeof foo == 'undefined') {
    do_something_else();
}
else {
    do_something();
}