Q130- What is the difference between null and undefined in JavaScript?

In JavaScript, both null and undefined represent absence of a value, but they are not the same thing — they mean different kinds of “nothing”.

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🎯 1. undefined

• It means a variable has been declared but not assigned a value.
• JavaScript automatically assigns undefined to uninitialized variables.
• It’s the default value of:
  • Uninitialized variables
  • Missing function arguments
  • Missing object properties

🧩 Example:

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

function greet(name) {
  console.log(name);
}
greet();         // undefined (no argument passed)

const obj = {};
console.log(obj.age); // undefined (property not found)

🧠 In short: undefined = “not yet assigned.”

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🎯 2. null

• It means intentional absence of any object value.
• You manually assign null to indicate “no value” or “empty.”

🧩 Example:

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

🧠 In short: null = “intentionally empty.”

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⚖️ Comparison:

console.log(undefined == null);  // true  (loose equality)
console.log(undefined === null); // false (strict equality)

• == performs type conversion, so they’re loosely equal.
• === checks type and value, so they’re different types:
  • typeof undefined → "undefined"
  • typeof null → "object"

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🧭 Quick Summary

Feature	undefined	null
Type	undefined	object
Set by	JavaScript	Developer
Meaning	Variable not assigned	Explicitly no value
Common causes	Missing value	Intentional empty value

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✅ Example Use Case:

let user = null; // means user intentionally has no data yet
let age;         // means age hasn’t been set yet

🧩 In short:

• undefined → JavaScript’s way of saying “no value assigned yet.”
• null → Developer’s way of saying “nothing here on purpose.”