Not Defined !== undefined

Here’s one of the most frequently asked interview questions: Why is Not Defined not equal to undefined? In this post, we will discuss this topic in detail and I will explain the differences between the two concepts.

Undefined vs. Not Defined

1. Undefined: A variable that has been declared but not initialized has a default value of undefined. This means that the variable exists in memory, but it doesn't have a value assigned to it yet.

2. Not Defined: A variable that has not been declared or is out of scope is considered not defined. This means that the variable doesn't exist in memory, and attempting to access it will result in a ReferenceError.

Code Example:

// Variable declaration and initialization
var x; // declared, but not initialized (undefined)
console.log(x); // Output: undefined

x = 5; // initialized
console.log(x); // Output: 5

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

Summary:

• Undefined means the variable has been declared in memory, but it doesn't have a value assigned to it yet.
• Not defined means the variable doesn't exist in memory or is out of scope.