waada.js

class Waada {
  #state;
  #value;
  #error;
  #onResolve;
  #onReject;

  constructor(completion) {
    this.#state = 'PENDING';
    completion((value) => this.#setResolved(value), (error) => this.#setRejected(error));
  }

  #setResolved(value) {
    if(this.#isInPendingState()) {
      this.#value = value;
      this.#state = 'FULFILLED';
      if(this.#onResolve) this.#onResolve(this.#value);
    }
  }

  #setRejected(error) {
    if(this.#isInPendingState()) {
      this.#error = error;
      this.#state = 'REJECTED';
      if(this.#onReject) this.#onReject(this.#error);
    }
  }

  #isInPendingState() {
    return this.#state === 'PENDING';
  }

  #setOnResolve(res, rej,onResolve) {
    return this.#onResolve = (value) => {
      try {
        res(onResolve(value))
      } catch(e) {
        const error = this.#onReject ? this.#onReject(e) : e;
        rej(error);
      }
    }
  }

  #setOnReject(rej, onReject) {
    this.#onReject = (error) => rej(onReject(error));
  }

  then(onResolve) {
    return new Waada((res, rej) => {
      this.#setOnResolve(res, rej, onResolve);
      if(this.#state === 'FULFILLED') this.#onResolve(this.#value);
    });
  }

  catch(onReject) {
    return new Waada((_, rej) => {
      this.#setOnReject(rej, onReject);
      if(this.#state === 'REJECTED') this.#onReject(this.#error);
    });
  }

  toString() {
    return `Value: ${this.#value}, Error: ${this.#error}, State: ${this.#state}`;
  }
}