TL;DR A ReferenceError occurs when trying to use an undeclared or undefined variable in JavaScript. Causes include typos, missing declarations, out-of-scope variables, and missing imports. To troubleshoot, check variable names, verify declarations, review scope, and inspect imports. Following best practices like using linters, consistent naming conventions, and understanding scoping rules can help avoid ReferenceErrors. Reference errors: When variables don't exist As a Fullstack Developer, you've likely encountered your fair share of errors in JavaScript. One type of error that can be particularly frustrating is the ReferenceError. In this article, we'll delve into what ReferenceErrors are, why they occur, and how to troubleshoot them.