MyPromise

MyPromise.js

class MyPromise {
  constructor(fn) {
    this.status = 'PENDING';

    const resolve = (data) => {
      if (this.status !== 'PENDING') return;

      this.status = 'FULLFILLED';
      this.value = data;
      this.onChange?.();
    };

    const reject = (data) => {
      if (this.status !== 'PENDING') return;

      this.status = 'REJECTED';
      this.value = data;
      this.onChange?.();
    };

    try {
      fn(resolve, reject);
    } catch (err) {
      reject(err);
    }
  }

  then(onResolve, onReject) {
    return new MyPromise((resolve, reject) => {
      const map = { FULLFILLED: [onResolve, resolve], REJECTED: [onReject, reject] };

      const onChange = () => {
        setTimeout(() => {
          const [callback, fallback] = map[this.status];

          if (typeof callback !== 'function') {
            fallback(this.value);
            return;
          }

          try {
            const result = callback(this.value);
            result instanceof MyPromise ? result.then(resolve) : resolve(result);
          } catch (err) {
            reject(err);
          }
        });
      };

      if (this.status === 'PENDING') {
        this.onChange = onChange;
      } else {
        onChange();
      }
    });
  }

  catch(onReject) {
    return this.then(null, onReject);
  }
}