The error “Uncaught ReferenceError: ___ is not defined” is one of the most common issues JavaScript developers face. It usually means you’re trying to use a variable that hasn’t been declared or isn’t accessible in the current context. Whether it’s due to a typo, a missing declaration, or a script loading out of order, this error can break your code and halt functionality. The good news? It’s usually straightforward to fix. In this guide, we’ll explore why this error occurs and walk you through simple, effective steps to resolve it — even if you’re new to JavaScript.

Introduction

So, what exactly is an “Uncaught ReferenceError: is Not Defined”? In plain English, it means that your JavaScript code is trying to use a variable, function, or object that simply doesn’t exist in the current scope or hasn’t been declared yet. Think of it like trying to refer to “that red car” when there are no red cars around for miles. Your code is pointing to something that isn’t there, and JavaScript, being the stickler for rules that it is, throws a fit.

This error is incredibly common, especially for beginners, but even seasoned developers can fall victim to it during late-night coding sessions or when refactoring complex applications. The good news is that once you understand the common culprits, debugging becomes much, much easier.

Common Causes of the Error

Let’s break down the usual suspects behind this annoying error:

1. Typo, Typo, Typo! This is hands down the most frequent reason. A simple misspelling of a variable name, function name, or object property can lead to this error. myVariable vs. myvariabl, handleClick vs. handleclick – you get the idea. Case sensitivity is a big deal in JavaScript!
2. Undeclared Variables: You might be trying to use a variable without ever having declared it with var, let, or const. For example, console.log(userName); without let userName = "Alice"; beforehand will definitely throw this error.
3. Scope Issues: JavaScript has something called “scope,” which determines where variables and functions are accessible. If you declare a variable inside a function, it’s usually only accessible within that function. Trying to use it outside will result in a ReferenceError
4. Incorrect Script Loading Order: If your HTML loads a JavaScript file that depends on another JavaScript file (e.g., a library like jQuery or a custom utility script), but the dependent file is loaded after the one trying to use it, you’ll see this error.
5. Using a Variable Before It’s Initialized (Temporal Dead Zone): With let and const, there’s a concept called the “Temporal Dead Zone.” If you try to access a let or const variable before its declaration line is executed, you’ll get a ReferenceError. This doesn’t apply to var in the same way (due to hoisting), but it’s good to be aware of.
6. Missing or Incorrect Imports/Exports (for modules): If you’re working with ES6 modules, forgetting to import a function or variable that’s exported from another file will cause this error.

How to Fix It (Step-by-Step)

Alright, time to get our hands dirty and fix this thing!

Identify the “Undefined” Element:

Look closely at the error message in your browser’s developer console. It will explicitly tell you what is not defined. For example: Uncaught ReferenceError: myFunctionName is not defined at <anonymous>:1:1. The myFunctionName part is your target.

Check for Typos:

Seriously, double-check the spelling of the variable, function, or object property name that’s causing the error. Pay attention to capitalization as JavaScript is case-sensitive (myVar is different from myvar). Even better, copy the name directly from where it’s supposed to be defined.

Verify Declaration:

Variables:

Did you declare the variable using var, let, or const before you tried to use it?

// BAD
userName = "Bob";
console.log(userName); // Uncaught ReferenceError: userName is not defined

// GOOD
let userName = "Bob";
console.log(userName);

Functions:

Is the function actually defined before you call it?

// BAD
sayHello();
function sayHello() {
  console.log("Hello!");
} // This might still work due to hoisting for function declarations, but it's bad practice and confusing.

// GOOD (better practice)
const sayHello = () => {
  console.log("Hello!");
};
sayHello();

// GOOD (traditional function declaration)
function greetUser() {
  console.log("Greetings!");
}
greetUser();

Inspect Scope:

If the variable/function is declared, is it accessible in the place where you’re trying to use it? Remember, variables declared inside functions usually stay within those functions. If you need to access it globally, declare it outside any function, or pass it as an argument.

function calculateTotal() {
  let price = 10;
  let quantity = 5;
  let total = price * quantity;
}
// console.log(total); // Uncaught ReferenceError: total is not defined (because it's inside calculateTotal)

Review Script Loading Order (for HTML files):

• Open your HTML file.
• Check the <script> tags.
• Ensure that any script that defines the problematic variable/function is loaded before the script that uses it.
• A common pattern is to put your custom JavaScript files at the end of the <body> tag, or use defer or async attributes on your <script> tags (but be mindful of their implications).

<script src="my_app.js"></script> <script src="jquery.js"></script>

<script src="jquery.js"></script>
<script src="my_app.js"></script>

Check Imports/Exports (if using modules):

If you’re working with import and export statements, make sure:

• The variable/function is correctly exported from its source file.
• You are correctly importing it into the file where you’re trying to use it

// myModule.js
export const API_KEY = "abc123";
export function fetchData() { /* ... */ }

// app.js
import { API_KEY, fetchData } from './myModule.js';
console.log(API_KEY);
fetchData();

Test the Fix

After you’ve made your changes, it’s crucial to test them.

1. Save your files.
2. Refresh your browser (a hard refresh, typically Ctrl + Shift + R or Cmd + Shift + R, can be helpful to clear cached scripts).
3. Open your browser’s developer console (F12 or right-click -> “Inspect” -> “Console” tab).
4. Look for the Uncaught ReferenceError. If it’s gone, congratulations! If not, go back to step 1 of “How to Fix It” and re-evaluate. You might have another instance of the error, or the fix you applied wasn’t the root cause.

Pro Tips

• Use debugger and console.log(): These are your best friends for debugging. Set breakpoints or strategically place console.log() statements to see the values of variables and the flow of your code.
• Linters (ESLint, Prettier): Use tools like ESLint and Prettier. They can catch many common errors, including undeclared variables and typos, before you even run your code.
• IDE Autocomplete: Modern Integrated Development Environments (IDEs) like VS Code have excellent autocomplete features. If your IDE isn’t suggesting the variable/function name you expect, it’s a good sign that it’s not defined in the current scope or has a typo.
• Small, Incremental Changes: When debugging, make one change at a time and then test. This helps you isolate the cause of the problem.
• Don’t Panic: This error is common. Take a deep breath, and systematically go through the steps.

Related Errors or Alternative Scenarios

While Uncaught ReferenceError: is Not Defined is specific, here are some related errors or scenarios that can seem similar:

• Uncaught TypeError: Cannot read properties of undefined (reading 'xyz'): This usually means you have an object, but one of its properties is undefined, and you’re trying to access a sub-property of that undefined property. For example, if user exists but user.address is undefined, then user.address.street would throw this TypeError.
• Uncaught TypeError: xyz is not a function: This means you’re trying to call something as a function, but it’s not actually a function (e.g., it’s a variable holding a string or a number).
• Variable Shadowing: While not an error itself, it can lead to unexpected behavior that feels like a ReferenceError. This happens when you declare a new variable with the same name as an outer-scope variable, effectively “shadowing” the outer one within the inner scope.

Conclusion

The “Uncaught ReferenceError: is not defined” error may look intimidating at first, but it’s one of the easier ones to troubleshoot once you understand what causes it. Most of the time, it’s due to a missing declaration, a typo, or a script load issue. By following the steps above, you’ll be well-equipped to fix this error — and prevent it in the future.