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undefined vs not defined in JavaScript

JavaScript handles variables in a unique way compared to many other programming languages, especially regarding the concepts of undefined and not defined.

❔ What is undefined?

In JavaScript, undefined is a special keyword and primitive value. It is automatically assigned to variables that have been declared but not yet given a value.

Why does this happen?

  • When JavaScript code runs, it first creates a global execution context.
  • During this process, JavaScript allocates memory for all variables and functions before any code is executed (this phase is called hoisting).
  • For variables, JavaScript sets their initial value to undefined as a placeholder until they are assigned a real value.
Example: Hoisting and undefined
console.log("a =", a); // Output: a = undefined
var a = 10;
console.log("a =", a); // Output: a = 10

Explanation

  • a is declared with var. Before the value 10 is assigned, a exists in memory with the value undefined.
  • After the assignment, a holds the value 10.

⚠ What is not defined?

A variable is not defined when you try to access a variable that has never been declared in your code.

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

Explanation

x has not been declared anywhere in the code. Trying to access it causes a ReferenceError.

🆚 Key Differences

undefined not defined
When? Variable declared but unassigned Variable never declared
Error? No error; value is undefined ReferenceError when accessed
Example var a; console.log(a); // undefined console.log(x); // ReferenceError: x is not defined

⚠ Common Mistake: Assigning undefined

It's possible to assign undefined to a variable:

var b = undefined; // Technically allowed, but discouraged

However, this is generally considered bad practice because:

  • undefined is meant as a placeholder before a variable is assigned a value.
  • Assigning undefined directly might make your code harder to understand and can lead to bugs or confusion.

Quick FAQs

  • Is undefined the same as "empty"?

    No. An undefined variable has memory reserved for it; it's not the same as empty or non-existent.

  • What if I declare a variable but never assign a value?

    It will hold the value undefined for its entire lifetime.

  • Should I use undefined as a value on purpose?

    Best practice is to let JavaScript use undefined naturally rather than assigning it yourself.

Key Takeaway

  • undefined: Variable declared, memory reserved, but not yet assigned a value.
  • not defined: Variable does not exist in the current scope; accessing it throws a ReferenceError.
  • Avoid directly assigning undefined to variables.