i18next services not initialising - "Cannot read properties of undefined (reading 'add')" at runtime when adding a custom formatter function

Question

All of a sudden I've started getting an error at runtime when using i18next (version 22.4.5 ) when executing the following code (at services.formatter.add ):

"Uncaught TypeError: Cannot read properties of undefined (reading 'add')"

Upon debugging, the services object on i18nextInstance is empty ( {} ) after initialisation (see screenshot) which would cause the TypeError, but I can't find out why this object is now empty.

const i18nextInstance = i18next.createInstance({
    debug: true,
    lng: currentLanguage,
    fallbackLng: 'en-GB', // Fallback to English strings and don't show an error
    fallbackNS: defaultNamespace, // Fallback for strings translated with `t`
    defaultNS: defaultNamespace,
    resources: resources
});

i18nextInstance.t('common:Hello World');

/**
 * Formatter function for locale-specific relative time strings 
 */
i18nextInstance.services.formatter.add('fromnow', (value: Date, lng, options): string => {
    return timeAgo(value, lng);
});

I was working with this exact version of the package last week and it was functioning as expected - what would cause this all of a sudden? My coworker ran the same code (we're both on Windows) and he had the exact same problem. I only encountered this when trying to get the component code Jest tests working, but now it's appearing in Chrome.

For context, I'm using this library inside a StencilJS project, inside its own module that is imported at the start of the app by some components.

I tried installing a later version of the library (22.4.9) with no luck.

I've tried resetting my node_modules folder and re-installing the library, as well as deleting all stencil build-related folders (WWW, dist, loader).

I did a hard reload on Chrome of the offending page.

Answer 1

Figured it out - turns out createInstance doesn't initialise the instance, so I needed to init add after createInstance :

i18nextInstance.init();

This is really weird as I didn't need to use that function before, but I guess there may have been something cached?