JavaScript null check

In JavaScript, null is a special singleton object which is helpful for signaling "no value". You can test for it by comparison and, as usual in JavaScript, it's a good practice to use the === operator to avoid confusing type coercion:

var a = null;
alert(a === null); // true

As @rynah mentions, "undefined" is a bit confusing in JavaScript. However, it's always safe to test if the typeof(x) is the string "undefined", even if "x" is not a declared variable:

alert(typeof(x) === 'undefined'); // true

Also, variables can have the "undefined value" if they are not initialized:

var y;
alert(typeof(y) === 'undefined'); // true

Putting it all together, your check should look like this:

if ((typeof(data) !== 'undefined') && (data !== null)) {
  // ...

However, since the variable "data" is always defined since it is a formal function parameter, using the "typeof" operator is unnecessary and you can safely compare directly with the "undefined value".

function(data) {
  if ((data !== undefined) && (data !== null)) {
    // ...

This snippet amounts to saying "if the function was called with an argument which is defined and is not null..."

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

A: Yes, data will be set to undefined. See section 10.5 Declaration Binding Instantiation of the spec. But accessing an undefined value does not raise an error. You're probably confusing this with accessing an undeclared variable in strict mode which does raise an error.

Q: 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.

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

A: Correct. Note that the following tests are equivalent:

data != null
data != undefined
data !== null && data !== undefined

See section 11.9.3 The Abstract Equality Comparison Algorithm and section 11.9.6 The Strict Equality Comparison Algorithm of the spec.

The simple way to do your test is :

function (data) {
    if (data) { // check if null, undefined, empty ...
        // some code here
    }
}

typeof foo === "undefined" is different from foo === undefined, never confuse them. typeof foo === "undefined" is what you really need. Also, use !== in place of !=

So the statement can be written as

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

Edit:

You can not use foo === undefined for undeclared variables.

var t1;

if(typeof t1 === "undefined")
{
  alert("cp1");
}

if(t1 === undefined)
{
  alert("cp2");
}

if(typeof t2 === "undefined")
{
  alert("cp3");
}

if(t2 === undefined) // fails as t2 is never declared
{
  alert("cp4");
}