Register Node Module Manually - node.js

Question

Question:
I have a project in TypeScript that uses several APIs I don't have access to on my computer (they exist on the web). The code will compile fine locally since I have all the APIs in foo.d.ts files, and so the system knows they exist somewhere.
However, I want to unit test parts of the code with a NodeJS app. I can import the code into node just fine, but whenever I reach code that imports a module from a definition file, I get the following error:
Error: Cannot find module 'messages'
at Function.Module._resolveFilename (module.js:527:15)
at Function.Module._load (module.js:476:23)
at Module.require (module.js:568:17)
at require (internal/module.js:11:18)
at Object.<anonymous> (~/dev/repos/sample_typescript_fail/App.js:3:18)
at Module._compile (module.js:624:30)
at Object.Module._extensions..js (module.js:635:10)
at Module.load (module.js:545:32)
at tryModuleLoad (module.js:508:12)
at Function.Module._load (module.js:500:3)
...
This makes sense, since that code is just defined locally, and does not exist.
Can I manually register modules to NodeJS, like
Registry.register('messages', () => {...});
so that I can compile and test with polyfills?
Here's an example app
package.json
{
"name": "sample_typescript_declare_issue",
"version": "1.0.0",
"description": "",
"main": "index.ts",
"scripts": {
"start": "ts-node index.ts"
},
"author": "",
"license": "MIT"
}
index.ts
import {App} from "./App";
console.log("Starting program");
// How do I fake "import {MessageSender} from "messages";"
// here so that I can run this node app as a test?
let app: App = new App();
console.log("Ending program");
App.ts
import {MessageSender} from "messages";
export class App {
constructor() {
let messageSender: MessageSender = new MessageSender();
messageSender.sendMessage("foo!");
}
}
node_modules/#types/messages/index.d.ts
export = Messages;
export as namespace Messages;
declare module Messages {
class MessageSender {
constructor();
sendMessage(message: any): void;
}
}
Running Example App
Running with npm start gives the error message above.
Running tsc *.tsc compiles just fine.
Other things I've tried
Updating package.json to include a bin:
{
"name": "sample_typescript_declare_issue",
"version": "1.0.0",
"description": "",
"main": "index.ts",
"scripts": {
"start": "ts-node index.ts"
},
"author": "",
"license": "MIT",
"bin": {
"messages": "./polyfills/messages/index.ts"
}
}

Answer 1

As you mentioned, compiling works fine - this is just a question of availability of .d.ts files.
What you want to do is alter module import at runtime, in other words alter the behaviour of the nodejs require function since
import {MessageSender} from "messages";
will be transpiled in javascript (ES6) to something like
const messages_1 = require("messages");
...
messages_1.MessageSender
To modify that behaviour, the first thing that springs to mind is to use the deprecated - but still available - require.extensions object.
When running locally you must first inject something like
require.extensions['.js'] = (module, filename) => {
if (filename === 'messages') {
// then load mock module/polyfill using the passed module object
// see (https://nodejs.org/api/modules.html#modules_the_module_object)
}
};
The doc says there are better alternatives but fails to clearly mention any.
Another possibility is to look at projects like sandboxed-module which should help (I have not tested it)