In JavaScript, what is the difference between an undeclared variable and an undefined variable?

Introduction / Context:
In JavaScript, variable handling can be confusing for beginners, especially when they encounter ReferenceError and values such as undefined. Understanding the difference between an undeclared variable and an undefined variable is important for debugging, writing safer code, and working with strict mode. Interviewers often ask this question to ensure that candidates know how the language treats variables at declaration and at runtime.

Given Data / Assumptions:

• The language is JavaScript running in a browser or in a runtime such as Node.js.
• We are comparing the concepts of undeclared and undefined for variables.
• We assume that the candidate is familiar with var, let, and const declarations.

Concept / Approach:
An undeclared variable is one that has never been declared in the current scope using var, let, or const. Attempting to access such a variable in strict mode produces a ReferenceError. An undefined variable, in contrast, is one that has been declared but has not yet been assigned any other value, so its value is the special primitive undefined. This difference affects error handling and type checks such as typeof.

Step-by-Step Solution:
Step 1: Consider a variable name that you never declared anywhere in your code and then try to use it. That is an undeclared variable. Step 2: Recognise that, especially in strict mode, using such a name throws a ReferenceError at runtime because the engine has no record of the identifier. Step 3: Now consider a variable that you declare using var, let, or const but do not assign a value to immediately. Step 4: When you read that variable before any assignment, its value is undefined, meaning that the variable exists in the scope but has not been initialised with a meaningful value. Step 5: Compare the options and choose the one that clearly states that undeclared refers to no declaration at all, while undefined refers to a declared variable with no value assigned.

Verification / Alternative check:
You can verify this by writing a short example. In strict mode, typing console.log(notDeclared) will cause a ReferenceError because notDeclared is undeclared. Declaring var x; and then running console.log(x) prints undefined, because x exists but has no assigned value. This simple experiment confirms the conceptual difference tested by the question.

Why Other Options Are Wrong:
Option B: Confuses the meanings and attributes arbitrary types such as constant and string, which do not define these terms. Option C: Suggests that undeclared and undefined are tied to strict versus non strict modes in a symmetric way, which is not how JavaScript works. Option D: Claims that there is no difference, but developers experience different behaviour and error messages when dealing with undeclared and undefined variables.

Common Pitfalls:
Developers sometimes mistakenly use variables without declaring them, which can create global properties on the window object in non strict mode and lead to hard to track bugs. Another pitfall is to assume that undefined means the same thing as null, although they represent different concepts. Using strict mode and always declaring variables helps avoid undeclared variable issues altogether.

Final Answer:
In JavaScript, An undeclared variable has never been declared in any scope, while an undefined variable has been declared but has not yet been assigned a value.