undefined vs not defined in JavaScript

undefined vs not defined in JavaScript

undefined and "not defined" look similar but are completely different. One is a value, the other is an error.

undefined Is a Value

undefined is a special value in JavaScript. It means a variable has been declared but has not been assigned a value yet.

let x;
console.log(x); // undefined
console.log(typeof x); // "undefined"

During the memory allocation phase, var variables are initialized to undefined. let and const are placed in the TDZ, but after the declaration line without an assignment, they are also undefined.

"not defined" Is an Error

"not defined" is what the engine says when a variable does not exist in any accessible scope. It throws a ReferenceError.

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

The variable y was never declared anywhere, so it is "not defined."

The Confusing Error Message

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

The error message says "z is not defined," which sounds like undefined. But it means z does not exist at all. The wording is a known source of confusion.

typeof and Safety

typeof on an undeclared variable returns "undefined" without throwing:

console.log(typeof w); // "undefined" (no error)

This is the safe way to check for a variable that may not exist (e.g., checking if a global API is available).

Common Sources of undefined

• A declared but unassigned variable.
• A function parameter that was not passed.
• A function with no return statement.
• Accessing a property that does not exist on an object.
• The result of void 0.

Common Sources of "not defined"

• Using a variable that was never declared.
• Misspelling a variable name.
• Using a variable outside its scope (e.g., a let outside its block).
• TDZ access for let/const (the message is slightly different: "Cannot access 'x' before initialization").

The Takeaway

undefined is a value meaning "declared but no value yet." "not defined" is a ReferenceError meaning "does not exist in any scope." typeof returns "undefined" for both but does not throw for undeclared variables, making it safe for existence checks.