Testing components that use TranslateService

v18 exposes ITranslateService — an abstract class that declares the shape of the public TranslateService API. It is re-exported from the main entry point (@ngx-translate/core) and is useful as a type contract for test fakes and library code that wants to depend on the API surface without pulling in the full service implementation.

Mock with a partial implementation

For most component tests you only need a few methods. Use Partial<ITranslateService> and cast through as unknown as ITranslateService when wiring up the provider:

import { signal } from "@angular/core";
import { TestBed } from "@angular/core/testing";
import {
    TranslateService,
    ITranslateService,
    Language,
} from "@ngx-translate/core";
import { of } from "rxjs";

const fakeTranslateService: Partial<ITranslateService> = {
    currentLang: signal<Language | null>("en"),
    fallbackLang: signal<Language | null>("en"),
    instant: (key: string | string[]) => `[${key as string}]`,
    get: (key: string | string[]) => of(`[${key as string}]`),
};

TestBed.configureTestingModule({
    providers: [
        { provide: TranslateService, useValue: fakeTranslateService },
    ],
});

The cast through unknown is needed because Partial<ITranslateService> is not assignable to ITranslateService directly — but at the provider boundary Angular only calls the methods your component actually uses, so a partial fake is safe at runtime.

Use the real service with setTranslation()

If your test exercises real translation lookup (interpolation, fallback chain), use the real TranslateService and seed it directly:

import { TestBed } from "@angular/core/testing";
import { TranslateService, provideTranslateService } from "@ngx-translate/core";
import { firstValueFrom } from "rxjs";

TestBed.configureTestingModule({
    providers: [
        provideTranslateService({ fallbackLang: "en" }),
    ],
});

const translate = TestBed.inject(TranslateService);
translate.setTranslation("en", {
    greeting: "Hello {{ name }}",
});
await firstValueFrom(translate.use("en"));

const text = translate.instant("greeting", { name: "World" });
// text === "Hello World"

This avoids mocking the interpolation logic and keeps the test honest.