Null, Undefined and “Not defined” in Javascript

In JavaScript, null, undefined, and not defined are three distinct concepts that refer to different values or states. In this article we shall look at null, undefined and not defined in Javascript and their unique use cases

Null

Null is the intentional absence of any object value. This could be a property of an object, or an expected value of a variable.

Example

let person = {
"name": "Lawson Luke",
"hobby": null
}
console.log(person.hobby); //null

When validating user inputs, values can also be checked for null values

let email = null;
if(email === null){
alert('Please enter your email');
}

Undefined

Undefined, in the simplest term, means that a property of variable is not initialized or does not exist.

let person = {
"name": "Lawson Luke",
"hobby": "coding",
}
console.log(person.age); //undefined

Not defined

“Not defined” refers to a variable that is not declared at any given point in time with the declaration keyword like var, let or const.

console.log(name); //Uncaught ReferenceError: name is not defined

It is important to note that null and undefined are distinct values, but they are both falsy values in JavaScript, which means they will evaluate to false in a Boolean context. Also, not defined is an error that occurs when you try to reference a variable or function that has not been declared

Conclusion

Now we know the difference between null, undefined and “not defined” in Javascript, and we can begin to write cleaner code and better logic.

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