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Creating a custom transform in Jest

I'm trying to create a custom transform for Jest, but running into a documentation roadblock which has me asking myself if I'm even on the right track.
Problem
I have a Rails project which is serving a Vue JS app. I want to write Jest tests for the JS app. In order to pass config variables from Rails to the app, I'm using ERB to template a small number of .js files. For example:
// in server-routes.js.erb
export default {
reports: '<%= Rails.application.config.relative_url_root %><%= Rails.application.routes.url_helpers.reports_path %>',
...
In my Webpack build for the Vue app, I use rails-erb-loader to preprocess the *.erb files before they get passed to the rest of the build process.
However, when I run my JS tests, Jest doesn't know anything about ERB loaders (reasonably enough). So my goal is to add a custom transform for Jest to convert the ERB files when running npm test.
Approach
I thought I might be able to use rails-erb-loader as a Jest transform:
// package.json
"jest": {
"moduleFileExtensions": [
"js",
"json",
"vue"
],
"moduleDirectories": [
"<rootDir>/node_modules"
],
"transform": {
".*\\.(vue)$": "vue-jest",
"^.+\\.js$": "babel-jest",
"^.+\\.js\\.erb$": "rails-erb-loader"
},
This doesn't work, however, because Jest transforms and Webpack loaders seemingly have different signatures. In particular, Jest expects a process function:
$ npm test
FAIL app/javascript/components/__tests__/dummy.test.js
● Test suite failed to run
TypeError: Jest: a transform must export a `process` function.
> 101 | import ServerRoutes from '../server-routes.js.erb';
| ^
at ScriptTransformer._getTransformer (node_modules/#jest/transform/build/ScriptTransformer.js:291:15)
at ScriptTransformer.transformSource (node_modules/#jest/transform/build/ScriptTransformer.js:353:28)
at ScriptTransformer._transformAndBuildScript (node_modules/#jest/transform/build/ScriptTransformer.js:457:40)
at ScriptTransformer.transform (node_modules/#jest/transform/build/ScriptTransformer.js:513:25)
at app/javascript/components/related-media.vue:101:1
at Object.<anonymous> (app/javascript/components/related-media.vue:232:3)
And this is where I get stuck, because I can't see where it's documented what the API and behaviour of a process function should be. In the documentation for the transform config option there's a single not very helpful example, and that's it as far as docs go, unless I've missed something.
I also note that babel-jest has a createTransformer function which sounds like it might be helpful, or at least illuminating, but again I can't find any docs on what it does.
If anyone has pointers on the details of creating custom Jest transforms, or at least some better docs, that would be great! Or, if I'm going about this the wrong way, what should I be doing?

You could look at ts-jest. https://github.com/kulshekhar/ts-jest/blob/master/src/ts-jest-transformer.ts. It is in typescript so is typed
Alternatively find the jest code that initiates the transform process. I don't think it is that difficult to find.
I think the transformer is either created with class constructor or via the factory function createTransformer.

From my understanding for the ts-jest-transformer and jest-erb-transformer it seems you need to export an object with public process or to export createTransformer methods which create a transformer object that have a process method.
a simple code example that runs before ts-jest
transform-example.js
const tsJest = require('ts-jest');
const t = tsJest.createTransformer();
module.exports = {
process(fileContent, filePath, jestConfig) {
const res = t.process(fileContent, filePath, jestConfig)
console.log(filePath);
return res;
}
}
jest.config.js
module.exports = {
transform: {
'^.+\\.tsx?$': ['<rootDir>/transform-example']
}
}
running this would run typescript tests (just like ts-jest) and log all the file paths transformed in the test.

Replacing dependencies in AMD module format with testdouble.js

I'm writing tests for a JS application using Jasmine and testdouble.js as a mocking library. I am using AMD format to organize code in modules, and RequreJS as a module loader. I was wondering how to use testdouble.js to replace dependency for the module being tested that is in AMD format and it is loading via RequireJS. The documentation is unclear about this or I am missing something, so if someone could point me in the right direction.
I'll post the example bellow that illustrates my setup and the problem that I am facing.
car.js
define("car", ["engine"], function(engine) {
function drive = {
engine.run();
}
return {
drive: drive
}
});
engine.js
define("engine", function() {
function run() {
console.log("Engine running!");
}
return {
run: run
}
});
car.spec.js
define(["car"], function(car) {
describe("Car", function() {
it("should run the motor when driving", function() {
// I am not sure how to mock the engine's object run method
// and where to place that logic, in beforeEach or...
td.replace(engine, "run");
car.drive();
// How to verify that when car.run() has executed, it calls this mocked method
td.verify(engine.run());
});
});
});

testdouble.js does not have any explicit support for AMD modules. The only module-related tricks it offers are Node.js specific and built on top of Node's CJS module loader.
What you would need to do in this case is require from the test a reference to engine and replace the run property, which it seems like you've done (your example is incomplete).
If you do this, don't forget to run td.reset() in an afterEach to restore the original properties to anything you replace!

jasmine getFixtures is not a function

I'm developing some unitTests using Jasmine. I'm currently working on Windows8.1 and when I run my test I get following error message.
TypeError: jasmine.getFixtures() is not a function
This is the code that I'm using. I don't know what I must change in order to do this work.
Karma.conf.js
//More configuration entries
...........
files: [
// app-specific code
'src/app/app.js',
// 3rd-party resources
'node_modules/angular-mocks/angular-mocks.js',
// test files
'test/unit/**/*.js',
//fixture to serve mockData in Tests
{ pattern: 'test/unit/mock-data/*.json' }
]
Here is the piece of code from my test.
it('Should load my fixture', function(){
jasmine.getFixtures().fixturesPath = 'HpIpsUi/test/unit/mock-data';
var json = readFixtures('diagnostics.json');
var result = JSON.parse(json);
expect(result).toBeDefined();1
})
It's not recognizing my call to the function jasmine.getFictures().
Thank you for your help

Have you included the jasmine-query library? It's defined there, not in the jasmine library.
Said jasmine-query.js needs to be downloaded, and added as file, because it's not installable with npm yet.

Using webpack to build a dependency before compiling the build

I'm using inline styles & my theme comes from a file called themes.json, which I create & inject into my build folder before calling webpack. I'd like webpack to handle this, too.
My first attempt was to use a plugin:
compiler.plugin('compilation', async(compilation, callback) => {
const themes = {}; // empty for this example
compilation.assets['themes.json'] = {
source: function() {
return themes;
},
size: function() {
return themes.length;
}
};
callback();
}
However, since plugins run async or parallel, I come across a race condition where my index.js that has a const themes = require('../build/themes.json') is trying to require something before it exists.
How can I make sure themes.json exists and is useable within my build? I see 3 possibilities:
There's a way to run plugins in serial that I don't know about
I somehow use a loader to perform this
I write a plugin that looks for require('../build/themes.json') and when it finds it, it creates & injects the themes.
Any help on the right way to do this?

Grunt and hood.ie test database

I'm currently running my test suite on AngularJS using Grunt, Karma, Jasmine and Protractor. The database library I'm using is hood.ie, which is a library on top of CouchDB. I start hood.ie using the following code in my Gruntfile:
hoodie: {
start: {
options: {
callback: function(config) {
grunt.config.set('connect.proxies.0.port', config.stack.couch.port);
}
}
}
},
However, I would like to have a separate database for running tests, which automatically resets afterwards. This way, the production data won't conflict with the tests.
How should I approach this? I would assume there's some kind of standard way of doing this, as I can imagine other people have come across the same problem, but I'm unable to find anything on the internet.

Currently, this seems to be impossible as the hoodie server does not support it. The best way to go about this is to modify it yourself at Hood.ie server Github repository by adding a parameter to define the folder in which the data will be stored, which is at the moment hardcoded to 'data' (https://github.com/hoodiehq/hoodie-server/blob/master/lib/core/environment.js#L48)
Something similar to this should work:
app_path: path.resolve(project_dir, argv.folder || 'data')

As the hoodie task is a 'multitask' you could have a test target in your hood.ie grunt task specific to testing, and then reference this in a grunt command used to run tests e.g:
hoodie: {
start: {
options: {
callback: function(config) {
grunt.config.set('connect.proxies.0.port', config.stack.couch.port);
}
}
},
test: {
options: {
callback: function(config) {
// Make test specific changes here.
}
}
}
}
// The task that runs tests first starting test deps. 'runtests' can be anything you want.
grunt.registerTask('test', 'Run unit tests', ['hoodie:test', 'runtests']);
Note: this will mean any other times you're referencing the hoodie task you'll need to be explicit as otherwise all the specified targets will be run. See this documentation on multitasks for more info. In this example you'd change hoodie to hoodie:start to run the 'start' task as previously defined.