Initializer decorator for Angular components (fully type-safe!)

ng-init.decorator.ts

import { Subscription } from 'rxjs';

const GLOBAL_COMPONENT_SUBSCRIPTIONS = new WeakMap<any, Set<Subscription>>();

export function NgInit<
    TargetType extends { constructor: Function, ngOnInit?(): void, ngOnDestroy?(): void },
    PropertyKey extends string,
    PropertyContainer extends { [k in PropertyKey]: () => (void | Subscription) },
    FunctionType extends PropertyContainer[PropertyKey]
>(
    target: TargetType & PropertyContainer,
    propertyKey: PropertyKey,
    _descriptor: TypedPropertyDescriptor<FunctionType>
) {

    const originalOnInit = target.ngOnInit;
    const originalOnDestroy = target.ngOnDestroy;

    target.ngOnInit = function(this: PropertyContainer) {
        if (originalOnInit) {
            originalOnInit.call(this);
        }

        const result = this[propertyKey].call(this);
        if (result instanceof Subscription) {
            let componentSubscriptions = GLOBAL_COMPONENT_SUBSCRIPTIONS.get(this);
            if (!componentSubscriptions) {
                componentSubscriptions = new Set<Subscription>();
                GLOBAL_COMPONENT_SUBSCRIPTIONS.set(this, componentSubscriptions);
            }
            componentSubscriptions.add(result);
        }
    };

    target.ngOnDestroy = function(this: PropertyContainer) {
        if (originalOnDestroy) {
            originalOnDestroy.call(this);
        }

        const componentSubscriptions = GLOBAL_COMPONENT_SUBSCRIPTIONS.get(this);
        GLOBAL_COMPONENT_SUBSCRIPTIONS.delete(this);

        componentSubscriptions?.forEach((subscription) => subscription.unsubscribe());
    };
}