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

It depends on the situation. If you're checking for something that may or may not have been defined globally outside your code (like jQuery perhaps) you want:

if (typeof(jQuery) != "undefined")

(No need for strict equality there, typeof always returns a string.) But if you have arguments to a function that may or may not have been passed, they'll always be defined, but null if omitted.

function sayHello(name) {
    if (name) return "Hello, " + name;
    else return "Hello unknown person";
}
sayHello(); // => "Hello unknown person"