UNDEFINED vs NOT DEFINED in JavaScript

When working with JavaScript, you may come across the terms “undefined” and “not defined”. It’s important to understand the difference between these two concepts in order to write clean and efficient code.

Undefined

In JavaScript, the value undefined represents a variable that has been declared but has not been assigned a value. For example:


var x;
console.log(x); // Output: undefined


In the above example, the variable x has been declared but has not been assigned a value, so its value is undefined. It’s important to note that undefined is a special value in JavaScript and is distinct from null, which represents the absence of a value.

Not Defined

On the other hand, when a variable is not defined at all, JavaScript throws a ReferenceError. For example:


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


In this case, the variable y has not been declared at all, so when we try to access it, JavaScript throws a ReferenceError because it cannot find a variable named y.

Conclusion

Understanding the difference between undefined and not defined in JavaScript is crucial for writing clean and error-free code. By properly declaring and initializing variables, you can avoid unexpected errors and ensure that your code runs smoothly.