RegeneratorRuntime is not defined

Node js Env - updated December 2015

This question has already been answered, please see accepted answer UNLESS running within NodeJS environment.

If like myself, you had the same error message: 'ReferenceError: regeneratorRuntime is not defined' but were running Babel within a NodeJS environment, then simply doing the following will likely solve your problem:

npm install babel-polyfill --save

Then insert the following require statement towards the top of the affected module to obtain required (generator) behaviour:

require("babel-polyfill");

This should be all you need, just importing the module adds required polyfill behaviour at runtime.

Similar to the post by arcseldon, I was running Babel within a NodeJS environment and getting the same error message 'ReferenceError: regeneratorRuntime is not defined'. While installing babel-polyfill does work, I went with @babel/plugin-transform-runtime instead.

@babel/plugin-transform-runtime

It needs to be installed in two ways ... first as a dev dependency:

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

and second as a production dependency:

npm install --save @babel/runtime

And then there needs to be one simple addition to your .babelrc file:

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

These additions give ES6 authoring functionality without the ReferenceError.

While I'm taking a different approach** to using Karma with Babel in my project, I suspect you're having the same problem I was: the Babel polyfill is not being loaded, and so you're not getting the functionality it supports (including the custom regenerator runtime that Babel uses to make generators work).

One approach would be to find a way to include the polyfill, perhaps by feeding it to Karma via the files array:

files: [
  'path/to/browser-polyfill.js', // edited: polyfill => browser-polyfill per P.Brian.Mackey's answer
  ...

An alternate approach may be to use Babel's runtime transformer [edit: on rereading the docs, this will not work unless you then browserify/webpack/etc. to process the require() calls created by the transformer]; per its docs,

The runtime optional transformer does three things:

• Automatically requires babel-runtime/regenerator when you use generators/async functions.
• Automatically requires babel-runtime/core-js and maps ES6 static methods and built-ins.
• Removes the inline babel helpers and uses the module babel-runtime/helpers instead.

I have no experience with this, but I suspect you would do so by including the optional: ['runtime'] option from the Babel docs in your babelPreprocessor config, viz.:

'babelPreprocessor': {
  options: {
    optional: ['runtime'],  // per http://babeljs.io/docs/usage/options/
    sourceMap: 'inline'
  },
...

(** I'm currently using jspm + jspm-karma + some config to get the Babel polyfill to load in SystemJS; ask if relevant and I'll expound.)