When using async/await or other modern JavaScript features with build tools like Parcel or older Babel configurations, you might see:

ReferenceError: regeneratorRuntime is not defined

Why it happens

Babel transpiles modern JavaScript (like async/await) into older JavaScript that can run in older browsers. For async/await, it uses a library called regenerator-runtime to polyfill generators.

The error occurs when Babel transforms your code but the runtime isn’t available.

Solution 1: Import the runtime

Add this import at the top of your main JavaScript file:

import 'regenerator-runtime/runtime'

This adds about 25KB to your bundle size.

Solution 2: Configure browserslist (recommended)

The better solution is to tell your build tool which browsers you’re targeting. If your target browsers already support async/await, Babel won’t transform it and you won’t need the polyfill.

Add a browserslist property to your package.json:

{
  "browserslist": [
    "last 1 Chrome version"
  ]
}

For development/testing, this is sufficient. For production, you might want broader support:

{
  "browserslist": [
    "last 3 and_chr versions",
    "last 3 chrome versions",
    "last 3 opera versions",
    "last 3 ios_saf versions",
    "last 3 safari versions"
  ]
}

Or use a date-based approach:

{
  "browserslist": [
    "since 2017-06"
  ]
}

The key is to specify browsers recent enough to support async/await natively (roughly browsers from 2017 onwards).

Checking browser support

You can see all valid browserslist values at: https://github.com/browserslist/browserslist

To see what browsers your query matches:

npx browserslist

Modern alternatives

If you’re starting a new project, consider using more modern build tools like:

• Vite: Uses native ES modules in development and esbuild for production
• esbuild: Extremely fast bundler that handles modern JavaScript well
• Bun: JavaScript runtime with built-in bundler

These tools typically handle modern JavaScript features without requiring runtime polyfills for reasonably modern browsers.

Lessons in this unit:

0:	Introduction
1:	"X is not a function" errors
2:	"document is not defined" error
3:	Cannot assign to read only property
4:	Parse failure: Unterminated string constant
5:	FormData multipart fetch issues
6:	▶︎ regeneratorRuntime is not defined
7:	Unexpected identifier in modules
8:	Unterminated string literal