RegeneratorRuntime is Not Defined – Causes and Solutions

The error “RegeneratorRuntime is not defined” is a common JavaScript issue developers encounter when using modern JavaScript features such as async/await or generator functions. This error usually appears in browser-based applications, frontend frameworks, or build environments where required polyfills are missing or not configured correctly.

This guide explains what the error means, why it happens, and how to fix it step by step in a clear and practical way.

What Does “RegeneratorRuntime Is Not Defined” Mean?

regeneratorRuntime is a helper library used internally by JavaScript transpilers such as Babel to support generator functions and async/await syntax in environments that do not natively support them.

When your code is transpiled, but the required runtime helper is not available at execution time, JavaScript throws the error:

ReferenceError: regeneratorRuntime is not defined

This typically happens in older browsers or misconfigured build setups.

Why Does the RegeneratorRuntime Error Occur?

This error occurs due to one or more of the following reasons:

1. You are using async/await or generators
2. Babel transpiles your code but does not include the runtime
3. Required polyfills are missing
4. The target browser does not support modern JavaScript features
5. The build configuration is incomplete or incorrect

When Do You Commonly See This Error?

You may encounter this error in scenarios such as:

• React, Angular, or Vue applications
• Jest or Mocha test environments
• Webpack or Rollup builds
• Legacy browsers (Internet Explorer, older mobile browsers)
• Node.js versions that lack full ES support

How to Fix “RegeneratorRuntime is Not Defined”?

npm install regenerator-runtime

import 'regenerator-runtime/runtime';

npm install --save-dev @babel/plugin-transform-runtime
npm install --save @babel/runtime

{

"plugins": ["@babel/plugin-transform-runtime"]

}

npm install core-js regenerator-runtime

import 'core-js/stable';
import 'regenerator-runtime/runtime';

{
"presets": [
["@babel/preset-env", {
"targets": {
"browsers": ["last 2 versions", "not dead"]
}
}]
]
}

fetchData()
.then(data => console.log(data))
.catch(error => console.error(error));

How to Fix This Error in React Projects?

In React apps (especially those created with custom Webpack or older setups):

1. Install regenerator-runtime
2. Import it once in index.js or main.js
3. Verify Babel configuration includes @babel/preset-env

Create React App usually handles this automatically, but custom configurations may require manual fixes.

How to Fix This Error in Jest Tests?

If the error occurs during testing:

Add this to your test setup file:

import 'regenerator-runtime/runtime';

And ensure Jest is using Babel properly with babel-jest.

Best Practices to Avoid This Error

1. Always configure Babel correctly
2. Use @babel/plugin-transform-runtime
3. Keep Node.js and dependencies up to date
4. Avoid mixing old and new build configurations
5. Test across different environments

Final Thoughts

The “RegeneratorRuntime is not defined” error is not a bug in your code—it’s a configuration issue related to missing runtime support for modern JavaScript features. Once you understand why it occurs, fixing it becomes straightforward.

By installing the proper runtime, configuring Babel correctly, and targeting the right environments, you can eliminate this error and ensure your application runs smoothly across all platforms.

If you’re working on a complex frontend or JavaScript-heavy application and need expert help configuring modern build pipelines, experienced development teams can help streamline your setup and avoid such issues altogether.