Upgrading from v20 to v21 breaks custom backend

[reZach]

🐛 Bug Report

Upgrading from v20.6.1 to v21.0.0 causes a custom backend to stop working. I reviewed the migration guide and did not see any changes that would imply a custom backend stops working. Roughly, this is the contents of my custom backend

class Backend {
    constructor(services, backendOptions = {}, i18nextOptions = {}) {
        this.init(services, backendOptions, i18nextOptions);   
    }

    init(services, backendOptions, i18nextOptions) {
        // init code
    }

    // Writes a given translation to file
    write(writeQueue) {
        // code
    }

    // Reads a given translation file
    requestFileRead(filename, callback) {
        // code
    }

    // Reads a given translation file
    read(language, namespace, callback) {
        // code
    }

    // Not implementing at this time
    readMulti(languages, namespaces, callback) {
        throw "Not implemented exception.";
    }

    // Writes a missing translation to file
    create(languages, namespace, key, fallbackValue, callback) {
        // code
    }
}
Backend.type = "backend";

export default Backend;

To Reproduce

git clone https://github.com/reZach/secure-electron-template.git
cd secure-electron-template
// change i18next dependency in package.json from ^20.6.1 to ^21.0.0
npm i
npm run dev

Expected behavior

I expect the following error will not be thrown; "Uncaught TypeError: Cannot read property 'use' of undefined"

Your Environment

• runtime version: node v16.13.0, electron v13.6.2
• i18next version: 21.0.0 (20.6.1 does not have this issue)
• os: Windows

[jamuhl]

'Cannot read property "use" of undefined' -> looks more like i18n is undefined -> check the import (might be const i18n = require("i18next");)

[adrai]

I don't know why you used .default, but this fixes it:

btw: whitelist does not exist anymore:

https://www.i18next.com/misc/migration-guide#removed-deprecated

[reZach]

Thanks @adrai / @jamuhl, I'll apply these changes!