difference between null and undefined in JavaScript?

Question

According to What is the difference between null and undefined in JavaScript?, null and undefined are two different objects (having different types) in Javascript. But when I try this code

var a=null;
var b;
alert(a==null);      // expecting true
alert(a==undefined); // expecting false
alert(b==null);      // expecting false
alert(b==undefined); // expecting true

The output of the above code is:

true
true
true 
true

Now as == only matches the value, I thought that both undefined and null must have the same value. So I tried:

alert(null) -> gives null

alert(undefined) -> gives undefined

I don't understand how is this possible.

Here is the demo.

Edit

I understand that === will give the expected result because undefined and null have different types, but how does type conversion work in Javascript in the case of ==? Can we do explicit type conversion like we do in Java? I would like to apply a manual type conversion on undefined and null.

Answer 1

You need to use the identity operator ===, not the equality operator ==. With this change, your code works as expected:

alert(a===null);      // true
alert(a===undefined); // false
alert(b===null);      // false
alert(b===undefined); // true

The reason the equality operator fails in this case is because it attempts to do a type conversion. undefined is of type undefined, and null is of type object; in attempting to compare the two, Javascript converts both to false, which is why it ends up considering them equal. On the other hand, the identity operator doesn't do a type conversion, and requires the types to be equal to conclude equality.

Edit Thanks to @user1600680 for pointing out, the above isn't quite correct; the ECMAScript specification defines the null-to-undefined as special case, and equal. There's no intermediate conversion to false.

---

A simpler example of type conversion is number-to-string:

console.log(5 == "5");    // true
console.log(5 === "5");   // false

---

The above answer has a good quote from Douglas Crockford's Javascript: The Good Parts:

[The "==" operator does] the right thing when the operands are of the same type, but if they are of different types, they attempt to coerce the values. the rules by which they do that are complicated and unmemorable.

If you don't believe that the rules are complicated and unmemorable, a quick look at those rules will disabuse you of that notion.

Answer 2

undefined and null have very different semantic meanings.

undefined typically means "There wasn't any reply" and null means "There was a reply and that reply was nothing."

For instance, if I created this object:

var gameState = {
  state: loaded,
  lastPlayer: null,
  lastScore: null
};

This doesn't mean "I don't know who the last player was" rather it means "there wasn't a last player."