What are undefined variables in JavaScript?

In JavaScript, a variable is considered "undefined" if it has not been assigned a value. This is a special value of type undefined, which indicates the absence of a value.

For example:

let a;
console.log(a); // undefined

The undefined value can appear in several cases:

• The variable is declared but not initialized.
• The function does not explicitly return a value — it returns undefined.
• Accessing a non-existent property of an object.

It is important not to confuse undefined with null — null means an intentional absence of a value, while undefined means a default absence of a value.

Also, keep in mind that during hoisting of variable declarations with var, the variable exists but is initialized to undefined until assigned a value.