JavaScript null check

Question

I've come across the following code:

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

I'm somewhat new to JavaScript, but, from other questions I've been reading here, I'm under the impression that this code does not make much sense.

---

In particular, this answer states that

You'll get an error if you access an undefined variable in any context other than typeof.

Update: The (quote of the) answer above may be misleading. It should say «an undeclared variable», instead of «an undefined variable».

As I found out, in the answers by Ryan ♦, maerics, and nwellnhof, even when no arguments are provided to a function, its variables for the arguments are always declared. This fact also proves wrong the first item in the list below.

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From my understanding, the following scenarios may be experienced:

• The function was called with no arguments, thus making data an undefined variable, and raising an error on data != null.

• The function was called specifically with null (or undefined), as its argument, in which case data != null already protects the inner code, rendering && data !== undefined useless.

• The function was called with a non-null argument, in which case it will trivially pass both data != null and data !== undefined.

Q: Is my understanding correct?

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I've tried the following, in Firefox's console:

--
[15:31:31.057] false != null
[15:31:31.061] true
--
[15:31:37.985] false !== undefined
[15:31:37.989] true
--
[15:32:59.934] null != null
[15:32:59.937] false
--
[15:33:05.221] undefined != null
[15:33:05.225] false
--
[15:35:12.231] "" != null
[15:35:12.235] true
--
[15:35:19.214] "" !== undefined
[15:35:19.218] true

I can't figure out a case where the data !== undefined after data != null might be of any use.

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.