Method then

packages/zone.js/lib/common/promise.ts:528–552

(
        onFulfilled?: ((value: R) => TResult1 | PromiseLike<TResult1>) | undefined | null,
        onRejected?: ((reason: any) => TResult2 | PromiseLike<TResult2>) | undefined | null,
      )

      }

      then<TResult1 = R, TResult2 = never>(
        onFulfilled?: ((value: R) => TResult1 | PromiseLike<TResult1>) | undefined | null,
        onRejected?: ((reason: any) => TResult2 | PromiseLike<TResult2>) | undefined | null,
      ): Promise<TResult1 | TResult2> {
        // We must read `Symbol.species` safely because `this` may be anything. For instance, `this`
        // may be an object without a prototype (created through `Object.create(null)`); thus
        // `this.constructor` will be undefined. One of the use cases is SystemJS creating
        // prototype-less objects (modules) via `Object.create(null)`. The SystemJS creates an empty
        // object and copies promise properties into that object (within the `getOrCreateLoad`
        // function). The zone.js then checks if the resolved value has the `then` method and
        // invokes it with the `value` context. Otherwise, this will throw an error: `TypeError:
        // Cannot read properties of undefined (reading 'Symbol(Symbol.species)')`.
        let C = (this.constructor as any)?.[Symbol.species];
        if (!C || typeof C !== 'function') {
          C = this.constructor || ZoneAwarePromise;
        }
        const chainPromise: Promise<TResult1 | TResult2> = new (C as typeof ZoneAwarePromise)(noop);
        const zone = Zone.current;
        if ((this as any)[symbolState] == UNRESOLVED) {
          (<any[]>(this as any)[symbolValue]).push(zone, chainPromise, onFulfilled, onRejected);
        } else {
          scheduleResolveOrReject(this, zone, chainPromise as any, onFulfilled, onRejected);
        }
        return chainPromise;
      }

      catch<TResult = never>(
        onRejected?: ((reason: any) => TResult | PromiseLike<TResult>) | undefined | null,