What's the difference between null and undefined?

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

// Variable declared but not initialized
let name;
console.log(name); // undefined
 
// Function returns nothing
function doNothing() {}
console.log(doNothing()); // undefined
 
// No such property in object
const user = {name: 'John'};
console.log(user.age); // undefined

// Explicitly set "nothing"
let name = null;
console.log(name); // null
 
// Clear value
let user = {name: 'John'};
user.name = null; // Explicitly clear
 
// Not found
function findUser(id) {
  // If user not found
  return null; // Explicitly "not found"
}

let value;
 
// ❌ Bad — can break
if (value === undefined) { /* ... */ }
 
// ✅ Good — safe
if (typeof value === 'undefined') { /* ... */ }
 
// ✅ Even better — short
if (value === void 0) { /* ... */ }

let value = null;
 
// ✅ Simply compare
if (value === null) { /* ... */ }
 
// ✅ Or check falsiness
if (value == null) { /* ... */ } // Works for both null and undefined

console.log(null == undefined);  // true (loose comparison)
console.log(null === undefined); // false (strict comparison)
 
console.log(null == 0);  // false
console.log(null == ''); // false
console.log(null == false); // false
 
console.log(undefined == 0);  // false
console.log(undefined == ''); // false
console.log(undefined == false); // false

// ✅ Didn't initialize variable
let name;
 
// ✅ Nothing to return
function process() {
  // Return nothing
}
 
// ✅ No property
const obj = {};
console.log(obj.nonExistent); // undefined

// ✅ Explicitly set "empty"
let user = null; // No user yet
 
// ✅ Clear value
user = {name: 'John'};
user = null; // Explicitly cleared
 
// ✅ Didn't find data
function findUser(id) {
  // Search in database
  if (!found) {
    return null; // Explicitly "not found"
  }
}

// ❌ Mistake — not understanding difference
let value1 = null;
let value2;
 
if (value1 == value2) {
  console.log('Same'); // true! But they're different things
}
 
// ✅ Correct — check precisely
if (value1 === null && value2 === undefined) {
  console.log('Different types of emptiness');
}

// ❌ Mistake — loose comparison
if (value == null) { /* ... */ } // Works for both null and undefined
 
// ✅ Correct — strict comparison
if (value === null) { /* ... */ } // Only for null