My minimal promise implementation

MyPromise.js

const PENDING = 0;
const FULFILLED = 1;
const REJECTED = 2;

class MyPromise {
    constructor(fn) {
        this.status = PENDING;
        process.nextTick(() => {
            fn(payload => {
                this.status = FULFILLED;
                this.result = payload;
            }, err => {
                this.status = REJECTED;
                this.result = err;
            })
        });        
        return this;
    }
    then(fn) {
        return this.onComplete(fn, FULFILLED);
    }
    catch(fn) {
        return this.onComplete(fn, REJECTED);
    }
    onComplete(cb, status) {
        return new MyPromise((resolve, reject) => {
          const timer = setInterval(() => {
              if (this.status === PENDING) {
                  return;
              }
              clearInterval(timer);
              const statusMatch = this.status === status;
              const errorFallThrough = !statusMatch && status === FULFILLED;
              if (errorFallThrough) {
                  reject(this.result);
                  return;
              }
              if (!statusMatch) {
                  return;
              }
              try {
                  const r = cb(this.result);
                  resolve(r);
              } catch(err) {
                  reject(err);
              }
          }, 1);
        });
    }
}