What happens if a variable is declared but not initialized?

When a variable is declared but not initialized, its value is undefined. In programming languages such as JavaScript, for example, when you declare a variable using the let, const, or var keywords without assigning it a value, the variable exists in memory, but it does not hold any meaningful data yet. Instead, it is said to have an "undefined" value, meaning that it has been declared but not yet assigned the necessary value to hold.

This concept is crucial for developers to understand because trying to use that variable before initialization could lead to errors in code execution or unexpected behavior. Furthermore, undefined is a specific type in JavaScript and other languages, allowing developers to check if a variable has been assigned a value or not.

In contrast, a variable initialized with null would indicate intentional absence of any value, which is different from being undefined. Therefore, understanding the distinction between undefined and null is important in effective programming. The variable being considered valid refers to the fact that it exists in the scope, but it's the undefined state that describes its lack of a set value at the time of declaration.