What is a real-world way of allowing a mocha test to stub/mock fs so I can test a function without it accessing the disk?

Question

So my code looks like this:

import FsAsyncFactory from '../fs/FsAsyncFactory'
import ObjectHelper from '../utils/ObjectHelper'
import Config from './Config'

export class ConfigHelper {

    // this is the function under test
    public static async primeCacheObj(): Promise<void> {

        const configFileObj: any = await ConfigHelper.getConfigFileObj()

    }

    private static configCacheObj: Config = null

    private static async getConfigFileObj(): Promise<any> {
        const fsAsync: any = FsAsyncFactory.getFsAsync()
        const fileContents: string = await fsAsync.readFileAsync('./config/hubnodeConfig.json')

        return JSON.parse(fileContents)
    }

}

export default ConfigHelper

What's the best way to allow my unit testing code to stop it from actually hitting the disk?

Should I be using something like InversifyJS to allow dependency injection? I'd have an init script that would setup the "defaults" for when the app is running normally then "override" those defaults in the tests?

Answer 1

In JavaScript, traditionally people have been using monkey patching to write unit tests.

I personally think that monkey patching is a bad practice and leads to unmaintainable code. I prefer dependency injection and that is why I created InversifyJS.

You can use InversifyJS to override bindings on unit tests.

This means that ConfigHelper will get an instance of FsAsync via dependency injection.

@injectable()
export class ConfigHelper {

    @inject("FsAsync") private readonly _fsAsync: FsAsync;

    // this is the function under test
    public static async primeCacheObj(): Promise<void> {

        const configFileObj: any = await ConfigHelper.getConfigFileObj()

    }

    private static configCacheObj: Config = null

    private static async getConfigFileObj(): Promise<any> {
        const json = await this_fsAsync.readFileAsync('./xxx.json')
        return JSON.parse(json)
    }

}

During the unit test you can replace the FsAsync type binding:

container.bind<FsAsync>("FsAsync")
         .toDynamicValue(() => FsAsyncFactory.getFsAsync());

To inject a mock:

container.rebind<FsAsync>("FsAsync")
         .toDynamicValue(() => ({
             readFileAsync: (path) => {
                 return Promise.resolve("{ hardCodedJson: "happy days!" }");
             }
         }));