Null and Undefined

In JavaScript, there are two special data types to indicate the value absence: null and undefined. You might wonder why special data types are needed to indicate absence and why two? Well, in JS, things are not so simple. In this topic, we will figure out the difference between null and undefined, and learn when they are used.

Null

null means that the variable was explicitly assigned an empty or non-existent value. If the variable is null, we know that it does not contain an acceptable number, string, or other data type:

let name = null; 
console.log(name); // null

The example above shows that the variable name is unknown or does not have a value for some reason.

Undefined

The undefined value is returned when a variable is declared but isn't set. Let's consider the following example:

let count; 
console.log(count); // undefined 

Here, as expected, the undefined value was output to the console.

undefined may also occur in cases when object properties do not exist:

let person = {
  age: 27
};

console.log(person.name); // undefined

In this case, undefined is returned because we tried to output to the console a property name that wasn't specified in our object.

But that's not all! The undefined value is also returned when a function has a missing parameter:

function getDetails(a) {
  console.log(a);
}

getDetails(); // undefined

In the example above, undefined is printed because the parameter a was missing in the written function.

Check data types

null and undefined literally mean no value, but they have different data types:

console.log(typeof null);       // object
console.log(typeof undefined);  // undefined

You expected that type null would return null, didn't you?

You were right if you thought so. In fact, null is not an object, but a separate data type. This strange behavior is an officially recognized error in the language. The mistake has existed for several decades. The reason no one fixes it is that thousands of sites created in JavaScript depend on this kind of wrong behavior. Fixing this bug can lead to many sites and popular web applications crashing. That's why we have nothing else to do but remember this peculiarity of the language and take it into account when writing scripts. We will talk more about objects as a separate data type next time.

Conclusion

Now you know what null and undefined have in common. You also learn about the differences between these values and their weird features. Try not to misuse these data types: use them wisely, and you can avoid many mistakes in your programs.