What is the Difference Between null and undefined in JavaScript?

When working with JavaScript, you'll frequently encounter both null and undefined. While they may seem similar, they have distinct meanings and use cases that every developer should understand.

The Key Differences

Practical Examples

let a;          // undefined (default)
let b = null;   // null (explicitly set)

console.log(typeof a);  // "undefined"
console.log(typeof b);  // "object" (historical bug)

When to Use Each

1. Use undefined when:

   • A variable is declared but not initialized
   • A function parameter isn't provided
   • A function has no return value

2. Use null when:

   • You want to explicitly indicate "no value"
   • You need to clear an object reference
   • Working with DOM elements that don't exist

Common Pitfalls

null == undefined   // true (loose equality)
null === undefined  // false (strict equality)

Number(undefined)   // NaN
Number(null)        // 0

Best Practices

1. Always use strict equality (===) for comparisons
2. Initialize variables to null rather than leaving them undefined
3. Use typeof checks for undefined, direct comparison for null

Understanding these differences will help you write more predictable JavaScript code and debug type-related issues more effectively.