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:

Accessing a non-existent object property:

Default return value of functions:

When to Use null

Explicitly setting a variable to indicate it has no value:

Representing missing or unknown data in objects:

Initializing variables for future object assignments:

4. Key Differences

Feature	undefined	null
Type	undefined	object
Default Value	Yes, when a variable is declared but unassigned	No, must be explicitly assigned
Boolean Conversion	false	false
Usage	Indicates uninitialized state	Explicitly assigned to indicate no value

5. Additional Code Examples

Checking for Undefined or Null

Handling null and undefined Safely

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.