Register Node Module Manually

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

1. 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)