Why JavaScript 'var null' throw an error but 'var undefined' doesn't?

In JavaScript, var null throws a syntax error because null is a reserved keyword, while var undefined works because undefined is not a reserved identifier.

Reserved Keywords in JavaScript

JavaScript has reserved keywords that cannot be used as variable names. These include:

null
true
false
if
for
while
function
var
let
const

Why 'var null' Throws an Error

When you try to declare a variable named null, JavaScript throws a syntax error:

// This will throw a SyntaxError
var null = "some value";

SyntaxError: Unexpected token 'null'

Why 'var undefined' Works

undefined is not a reserved keyword in JavaScript. It's a global property, but you can still use it as a variable name (though it's not recommended):

var undefined = "Hello World";
console.log(undefined);

Hello World

Understanding null vs undefined

null represents an intentional absence of value, while undefined indicates a variable has been declared but not assigned a value:

var a;
var b = null;

console.log("a:", a);
console.log("b:", b);
console.log("typeof a:", typeof a);
console.log("typeof b:", typeof b);

a: undefined
b: null
typeof a: undefined
typeof b: object

Best Practices

Avoid using undefined as a variable name even though it's allowed. It can lead to confusing code and unexpected behavior. Instead, use meaningful variable names:

// Bad practice
var undefined = "some value";

// Good practice
var userInput = "some value";
var result = null; // Explicitly setting to null is fine

// No output - just showing proper syntax

Conclusion

var null fails because null is a reserved keyword, while var undefined works because undefined is not reserved. However, avoid using undefined as a variable name for code clarity.