simple-promise.ts

class MyPromise<T> {
  status: "PENDING" | "FULFILLED" | "REJECTED" = "PENDING";
  subjects: { complete: (value: T) => void }[] = [];
  result?: T;
  constructor(executor: (resolve: (result: MyPromise<T> | T) => void) => void) {
    const resolver = (result: MyPromise<T> | T) => {
      if (result instanceof MyPromise) {
        result.then(resolver);
      } else {
        this.result = result;
        this.status = "FULFILLED";
        this.subjects.forEach((i) => i.complete(result));
      }
    };
    executor(resolver);
  }

  then<NT = T>(onFulfilled: (value: T) => NT | MyPromise<NT> | void): MyPromise<NT> {
    const subject = {
      subscriber: undefined,
      complete(value: T) {
        const nextRes = onFulfilled(value);
        this.subscriber && this.subscriber(nextRes);
      },
    };
    this.subjects.push(subject);
    if (this.status === "FULFILLED") {
      queueMicrotask(() => subject.complete(this.result));
    }
    return new MyPromise((resolve) => {
      subject.subscriber = resolve;
    });
  }
}