Differences between Null and Undefined in JavaScript

When a variable is declared but not assigned a value, it is automatically (implicitly) initialized with undefined.

let name; // Variable is Declared But Not Assigned a Value
console.log(name); // Output: undefined

name = "John"; // Assigning a Value (John) to the name Variable
console.log(name); // Output: John

If a function does not have a return statement, it will return undefined by default.

function sayHi() {
    console.log("HI");
}

const data = sayHi();
console.log(data); // Output: undefined

When you access a property that doesn’t exist in an object, JavaScript will return undefined to indicate that the property is not present or hasn’t been assigned a value.

const user = {
    name: "John",
    age: 23,
    email: "dd",
};

console.log(user.email); // Output: undefined

/**
 * Use `in` operator or hasOwnProperty() to check
 * whether property is exist
 */
if ("email" in user) {
    console.log(user.email);
}

An explicit assignment of undefined is possible but generally not recommended, as it can be misleading and it is better to rely on JavaScript’s default behavior for uninitialized variables.

let user1 = undefined; // ❌
let user2 = null; // ✔️ use `null` instead.

null is a value that needs to be assigned explicitly to indicate the intentional absence of a value or emptiness.

let user = null; // Empty variable

JavaScript implicitly converts null to 0 when performing arithmetic operations.

console.log(null + 2); // 2 because 0 + 2 = 2
console.log(null * 5); // 0 because 0 * 5 = 0