JavaScript null check

Question

I\'ve come across the following code:

function test(data) {
    if (data != null && data !== undefined) {
        // some code here
    }
}
         


        

Answer 1

An “undefined variable” is different from the value undefined.

An undefined variable:

var a;
alert(b); // ReferenceError: b is not defined

A variable with the value undefined:

var a;
alert(a); // Alerts “undefined”

When a function takes an argument, that argument is always declared even if its value is undefined, and so there won’t be any error. You are right about != null followed by !== undefined being useless, though.