JavaScript check if variable exists (is defined/initialized)

Question

Which method of checking if a variable has been initialized is better/correct? (Assuming the variable could hold anything (string, int, object, function, etc.))

Answer 1

My preference is typeof(elem) != 'undefined' && elem != null.

However you choose, consider putting the check in a function like so

function existy (x) {
    return typeof (x) != 'undefined' && x != null;
}

If you don't know the variable is declared then continue with typeof (x) != 'undefined' && x != null;

Where you know the variable is declared but may not be existy, you could use

existy(elem) && doSomething(elem);

The variable you are checking may be a nested property sometimes. You can use prop || {} to go down the line checking existance to the property in question:

var exists = ((((existy(myObj).prop1||{}).prop2||{}).prop3||{})[1]||{}).prop4;

After each property use (...' || {}').nextProp so that a missing property won't throw an error.

Or you could use existy like existy(o) && existy(o.p) && existy(o.p.q) && doSomething(o.p.q)