How to Accurately Detect Null vs Undefined in JavaScript

In JavaScript, null and undefined both represent the absence of a value, but they serve different purposes and have distinct behaviors.

1. Definition

Undefined (undefined):

• undefined is a primitive type that signifies a variable has been declared but not assigned a value.
• If you access an object property that does not exist, it returns undefined.
• Functions that do not return a value explicitly return undefined.

Null (null):

• null is an object that explicitly represents the absence of a value.
• It is often used to intentionally clear a variable or indicate that a property has no value.

2. Type Differences

• typeof undefined returns 'undefined'.
• typeof null returns 'object' (this is a historical bug in JavaScript, but it remains for compatibility reasons).

3. Usage Scenarios

When to Use undefined

Declaring a variable without assigning a value:

1 let a;
2 console.log(a); // Output: undefined

Accessing a non-existent object property:

1 const obj = {};
2 console.log(obj.property); // Output: undefined

Default return value of functions:

1 function test() {}
2 console.log(test()); // Output: undefined

When to Use null

Explicitly setting a variable to indicate it has no value:

1 let b = null;
2 console.log(b); // Output: null

Representing missing or unknown data in objects:

1 const user = {
2   name: "Alice",
3   age: null, // Age might be unknown
4 };

Initializing variables for future object assignments:

1 let element = null; // Will later be assigned a DOM element

4. Key Differences

5. Additional Code Examples

Checking for Undefined or Null

1 let value;
2 if (value === undefined) {
3   console.log("Value is undefined");
4 }
5 
6 let data = null;
7 if (data === null) {
8   console.log("Data is null");
9 }

Handling null and undefined Safely

1 function getValue(input) {
2   return input ?? "Default Value";
3 }
4 
5 console.log(getValue(undefined)); // Output: Default Value
6 console.log(getValue(null)); // Output: Default Value
7 console.log(getValue("Hello")); // Output: Hello

Conclusion

Both null and undefined indicate an absence of value in JavaScript, but undefined is typically used by the JavaScript engine for uninitialized variables and missing properties, while null is used intentionally by developers to represent an empty or non-existent value. Understanding their distinctions helps prevent bugs and improves code readability.