Mocha - Running test ReferenceError: regeneratorRuntime is not defined - node.js

Question

I am trying to run tests with async/await using mocha. The project architecture was setup before I started working on it and I have been trying to update it's node version to 8.9.4. The project is an isomorphic application and uses babel, gulp and webpack to run.
To run the tests we run a gulp task. There are two .bablerc files in the project. One in the root folder of the project and another in the test fodler.
Both have the same configuration:
{
"presets": [
["env", {"exclude": ["transform-regenerator"]}],
"react",
"stage-1"
],
"plugins": [
"babel-plugin-root-import"
]
}
When I run the app locally there is no error returned anymore. However when I run the tests with gulp test:api I constantly get the error: ReferenceError: regeneratorRuntime is not defined
This is my gulp file in the test folder:
var gulp = require('gulp')
var gutil = require('gulp-util')
var gulpLoadPlugins = require('gulp-load-plugins')
var plugins = gulpLoadPlugins()
var babel = require('gulp-babel')
require('babel-register')({
presets:["es2015", "react", "stage-1"]
});
// This is a cheap way of getting 'test:browser' to run fully before 'test:api' kicks in.
gulp.task('test', ['test:browser'], function(){
return gulp.start('test:api')
});
gulp.task('test:api', function () {
global.env = 'test'
gulp.src(['test/unit-tests/server/**/*.spec.js'], {read: false})
.pipe(plugins.mocha({reporter: 'spec'}))
.once('error', function (error) {
console.log(error)
process.exit(1);
})
.once('end', function () {
process.exit(0);
})
});
gulp.task('default', ['test']);
Any help on why this is happening wouldd be much appreciated.

Answer 1

Node version 8 already has support for async/await so you do not need Babel to transform it; indeed, your root .babelrc includes this preset to exclude the regenerator that would transform async/await (and introduce a dependency on regeneratorRuntime):
["env", {"exclude": ["transform-regenerator"]}]
However, in your test file, the configuration does not specify this preset. Instead, it specifies the preset "es2015", which does include the unwanted transform-regenerator (as you can see at https://babeljs.io/docs/plugins/preset-es2015/). If you change this to match the presets in the root .babelrc, you'll get more consistent results.

Answer 2

Strangely i ran into this issue after i upgraded to Node v8.10.0 from v8.6.x . I had used babel-require like so in my test-setup.js
require('babel-register')();
and the testing tools are Mocha,chai,enzyme + JSDOM . I was getting the same issue when i was making a async call to a API, also while using generator functions via sagas. Adding babel-polyfill seemed to have solved the issue.
require('babel-register')();
require('babel-polyfill');
i guess even babel docs themselves advocate using polyfill for generators and such
Polyfill not included
You must include a polyfill separately when using features that require it, like generators.

Answer 3

Ran into the same issue when running mocha tests from within Visual Studio Code.
The solution was to add the necessary babel plugins in the Visual Studio Code settings.json :
"mocha.requires": [
"babel-register",
"babel-polyfill"
],

Answer 4

I've run into this error before myself when using async/await, mocha, nyc, and when attempting to run coverage. There's never an issue when leveraging mocha for running tests, just with mocha tests while leveraging nyc for running coverage.
11) Filesystem:removeDirectory
Filesystem.removeDirectory()
Should delete the directory "./tmp".:
ReferenceError: regeneratorRuntime is not defined
at Context.<anonymous> (build/tests/filesystem.js:153:67)
at processImmediate (internal/timers.js:461:21)
You can fix the issue a couple of different ways.
Method 1 - NPM's package.json:
...
"nyc": {
"require": [
"#babel/register",
"#babel/polyfill"
],
...
},
...
It really depends which polyfill package you're using. It's recommended to use the scoped (#babel) variant: #babel/pollyfill. However, if you're using babel-polyfill then ensure that's what you reference.
Method 2 - Direct Import
your-test-file.js (es6/7):
...
import '#babel/polyfill';
...
OR
your-test-file.js (CommonJS):
...
require("#babel/polyfill");
...
Don't assign it to a variable, just import or require the package. Again, using the package name for the variant you've sourced. It includes the polyfill and resolves the error.
HTH