A simple JavaScript Promise implementation for education purposes

PromiseSimple.js

class PromiseSimple {
  constructor(executionFunction) {
    this.promiseChain = [];
    this.handleError = () => {};

    this.onResolve = this.onResolve.bind(this);
    this.onReject = this.onReject.bind(this);

    // executing this function either calls this.onResolve
    // or this.onReject
    executionFunction(this.onResolve, this.onReject);
  }

  then(onResolve) {
    // appends provided function to the promiseChain list of this class
    // and returns the class instance
    this.promiseChain.push(onResolve);

    return this;
  }

  catch(handleError) {
    // sets the handleError method of this class to
    // the provided function and returns the class instance
    this.handleError = handleError;

    return this;
  }

  onResolve(value) {
    let storedValue = value;

    try {
      // every call of the .then() method on the promise
      // passes in a function that is appended to this.promiseChain
      // here we loop through the list of functions in this.promiseChain,
      // execute it, using the return of the execution as the input to
      // the next function in the list.  
      //
      // Can also be done with reduce
      /* 
      this.promiseChain.reduce( 
       (valReduced, nextFcn) => nextFcn(valReduced), // return of nextFcn execution is input of next call
       value // initial value
      )
      */
      this.promiseChain.forEach((nextFunction) => {
         storedValue = nextFunction(storedValue);
      });
    } catch (error) {
      this.promiseChain = [];

      this.onReject(error);
    }
  }

  onReject(error) {
    // calls this.handleError, which is set when the user calls the catch() method
    this.handleError(error);
  }
}