Snippet

Type-Safe Dependency Injection Tokens with Factory Providers

An `InjectionToken<T>` carries its type parameter all the way through `inject()`, so `useFeatureFlags()` returns a fully-typed `FeatureFlags` object with no casting anywhere in the chain. The `factory` option lets the token supply its own default when nothing else provides it, and because that factory itself calls `inject(RAW_CONFIG, { optional: true })`, the token can depend on another optional token without forcing every consumer to configure both. This pattern — a typed token whose factory composes other optional tokens — replaces the classic `@Optional() @Inject(TOKEN)` constructor boilerplate with a single declaration point that stays type-safe under strict mode.

snippet.js

javascript

import { InjectionToken, inject } from '@angular/core';
 
interface FeatureFlags {
  readonly darkMode: boolean;
  readonly betaCheckout: boolean;
}
 
const RAW_CONFIG = new InjectionToken<Partial<FeatureFlags>>('RAW_CONFIG');
 
const FEATURE_FLAGS = new InjectionToken<FeatureFlags>('FEATURE_FLAGS', {
  providedIn: 'root',
  factory: () => {
    const raw = inject(RAW_CONFIG, { optional: true });
    return {
      darkMode: raw?.darkMode ?? false,
      betaCheckout: raw?.betaCheckout ?? false,
    };
  },
});
 
function useFeatureFlags(): FeatureFlags {
  return inject(FEATURE_FLAGS);
}

angular

Breakdown

const FEATURE_FLAGS = new InjectionToken<FeatureFlags>('FEATURE_FLAGS', { ... })

The generic parameter binds the token's type to `FeatureFlags` for every future `inject(FEATURE_FLAGS)` call site.

factory: () => { const raw = inject(RAW_CONFIG, { optional: true }); ... }

Runs inside an injection context, so it can itself call `inject()` to compose values from other tokens rather than hardcoding defaults.

raw?.darkMode ?? false

Nullish coalescing supplies a concrete default only when `RAW_CONFIG` was never provided, keeping the returned object's fields non-optional.

function useFeatureFlags(): FeatureFlags { return inject(FEATURE_FLAGS); }

A typed injection wrapper usable in any injection context (constructors, field initializers, `provideAppInitializer`), returning `FeatureFlags` with no cast.

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Type-Safe Dependency Injection Tokens with Factory Providers

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