What are the differences between null, undefined, and undeclared?

Differences Between null, undefined, and undeclared

undefined

A variable that has been declared but not yet assigned a value:

let a;
console.log(a); // undefined

function foo(x) { return x; }
foo(); // undefined (no argument passed)

null

An intentionally assigned empty value meaning "no object" or "no value":

let user = null; // explicitly means no user currently
console.log(typeof null); // 'object' (historical bug in JS)

undeclared

A variable that has never been declared — accessing it throws a ReferenceError:

console.log(b); // ReferenceError: b is not defined

Use typeof to safely check for undeclared variables:

typeof b; // 'undefined' (no error thrown)

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Comparison Table

Feature	null	undefined	undeclared
Type	object	undefined	ReferenceError
Meaning	Intentionally empty	Declared but not assigned	Never declared
How it occurs	Manually assigned	Default value	—
Equality	null == undefined → true	null === undefined → false	—