JavaScript
Difference between null and undefined
Learn the difference between empty and undefined values and when to use each.
What is undefined?
In JavaScript, if you declare a variable but do not assign a value, that variable automatically holds undefined. The word means not defined: the variable exists but it has no value inside.
For example, let ad; declares the ad variable but its value is undefined. If you then write console.log(ad); the console shows undefined. This happens when you try to read a variable before assigning to it.
What is null?
null is an intentionally given empty value. You are saying the variable will be empty; JavaScript did not forget anything, you said empty for now.
For example, if a user has not yet picked a city you write let sehir = null;. If you then write console.log(sehir); the console shows null. So null says the variable has a value, and that value is empty.
When to use null, when undefined?
undefined is usually a state JavaScript itself produces: you forgot something and the system says no value. null is a value you write on purpose: empty for now, I will fill it later.
In short: when you want to give a variable an empty for now value, use null. When you forget to assign a value, JavaScript automatically gives undefined. In practice you do not write undefined by hand; you write null to mean I meant empty.
Exercise Show undefined first, then null
Declare a pick variable with let but do not assign a value (so it becomes undefined). Then assign null to pick. In both states, print its value with console.log.
Show example solution
let pick;
console.log(pick);
pick = null;
console.log(pick);
Test yourself
-
Which of these snippets prints undefined to the console?
- let yas = 30;
console.log(yas);
- let yas = null;
console.log(yas);
- let yas = "30";
console.log(yas);
- let yas;
console.log(yas);
Show answer
let yas;
console.log(yas); — Correct. yas is declared but never given a value, so it is undefined.
-
Assign null to pick
let pick = ____;
Show answer
let pick = null;
-
Match each value to its meaning
- undefined
- No value assigned
- null
- Intentional empty
- 42
- A number
- true
- Logical true
Next lesson: The typeof operator
