What is undefined and null in JavaScript?

In JavaScript, undefined and null are two distinct types that represent the absence of a value, but they are used in different contexts and have different meanings. Let’s break it down clearly:

1. undefined

• Type: undefined
• Meaning: A variable that has been declared but not assigned any value is undefined.
• Automatically assigned: JavaScript automatically assigns undefined to variables that are declared but not initialized, function parameters that are missing, or properties that don’t exist in an object.

Examples:

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

function test(x) {
  console.log(x);
}
test(); // undefined, because no argument was passed

let obj = {};
console.log(obj.name); // undefined, property doesn't exist

2. null

• Type: object (yes, confusingly typeof null returns "object")
• Meaning: null is an intentional absence of any value. You assign it to a variable to indicate “no value” explicitly.
• Used by developers to denote that a variable should be empty.

Examples:

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

let user = {
  name: "John",
  age: null // age is intentionally set to null
};
console.log(user.age); // null

3. Key Differences

✅ Summary:

• Use undefined when a value is missing automatically (JS default).
• Use null when you want to explicitly indicate that a variable is empty.