One of the most common JavaScript errors: function test() { let x = 10; } console.log(x); // ReferenceError At first glance, it looks simple. But this error is directly tied to how scope works in JavaScript. Here’s the issue: x is declared inside the function Its scope is limited to that function Accessing it outside → not allowed So JavaScript throws: “x is not defined” This isn’t just an error — it’s a fundamental concept that’s frequently tested in interviews. Understanding why it happens matters more than just recognizing it. 👉 I’ve explained this step-by-step (with interview context) in the full video (link in comments)