Difference between undefined value and null value

In JavaScript, undefined and null don’t mean the same thing. This article aims to explain the real meaning with examples. Hope you find it useful.

What is null?

null is a falsy assignment value. You as a programmer may want to assign the value null to a variable. It simply means the value is blank or non-existent.

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

Interestingly, when we use typeof to check the type of null, it returns "object".

typeof(null); // prints, "object"

Note: This can be confusing as null is a primitive value. This is a bug in the language but has not been corrected even after series of updates to the language

What is undefined?

undefined typically means a variable has been declared but has not yet been assigned a value.It is also a falsy value.

let name;
console.log(name); // prints, undefined

In the above example, we have declared a variable name but haven't assigned a value to it. Hence the variable name is undefined.

There is also a way to confirm it,

typeof(name); // prints "undefined"

When you access non-existent object properties, you get an undefined.

let information= {'name': 'James..'};
console.log(information.name); // prints, "James"
console.log(information.address); // prints, undefined

ReferenceError

In the case of undefined, the variable must be declared. On the contrary, accessing a variable that's not been declared will cause a ReferenceError.

console.log(Zuri);

If you haven’t declared zuri variable and trying to access it like shown above, you will get an error,

In Summary

To summarize,

• undefined and null are primitive values and they represent falsy values.
• undefined value is typically set by the JavaScript engine when a variable is declared but not assigned with any values.
• null value is typically set by programmers when they want to assign an empty/blank value.
• If you try to access the value of a variable that is not even declared, it will result in a ReferenceError.