Promise polyfill implementtion

promise.js

class MyPromise {
    constructor(cb) {
      this.value;
      this.state;
      this.fulfilledCB = [];
      this.rejecteddCB = [];
      cb(this.resolve, this.reject);
    }

    then = (successCB, failureCB) => {
      let nextVal;

      if (successCB && this.fulfilledCB.indexOf(successCB) === -1) {
        this.fulfilledCB.push(successCB);
      }

      if (failureCB && this.rejecteddCB.indexOf(failureCB) === -1) {
        this.rejecteddCB.push(failureCB);
      }

      if (this.state === 'fulfilled') {
        if (this.value && this.fulfilledCB.length > 0) {
          nextVal = this.fulfilledCB.splice(0,1)[0](this.value);

          if (this.fulfilledCB.length > 0) {
          this.resolve(nextVal);  
          }
        }
      } else if (this.state === 'rejected') {
        if (this.value && this.rejecteddCB.length > 0) {
          nextVal = this.rejecteddCB.splice(0,1)[0](this.value);

          if (this.rejecteddCB.length > 0) {
          this.reject(nextVal);  
          }
        }
      } 
      return this;
    }
    
    resolve = (value) => {
      if (this.state === 'rejected') return;
      this.state = 'fulfilled';
      this.value = value;

      if (this.fulfilledCB[0]) {
        this.then(this.fulfilledCB[0], null);  
      }
    };
      
    reject = (value) => {
      if (this.state === 'fulfilled') return;
      this.state = 'rejected';
      this.value = value;

      if (this.rejecteddCB[0]) {
        this.then(null, this.rejecteddCB[0]);
      }
    };
}