consume.js

import SimplePromise from 'CustomPromise'; 

// consuming a ES6 promise: 
function doSomething() {
  return new SimplePromise((resolve, reject) => {
    let condition = true; 
    if (condition) return resolve(resolveArgument); 
    return reject(rejectArgument); 
  });
}

doSomething.then((resolveArgument) => {
  // do something with resolveArgument
  console.log(resolveArgument);
}).catch((rejectArgument) => {
  if (rejectArgument) throw rejectArgument;
});

CustomPromise.js

export default class SimplePromise {
  constructor () {
    this.status = 'pending'; // 3 states pending, resolved, rejected
    this.result = null; // initially set to null
    this.callbacks = [];
  }
  
  then(onResolved, onRejected) {
    if (this.status === 'resolved') 
      return this.resolve(onResolved, onRejected);
    else 
      this.callbacks.push({
        onResolved: onResolved, 
        onRejected: onRejected
      });
  }
  
  resolve(value) {
    if (this.status === 'rejected') {
      this.onRejected(this.result);
    } else {
      this.onResolved(this.result);
    }

    return this;
  }
  
  onResolved(result) {
    if(this.status === 'resolved') return;
    this.status = 'resolved';
    release(result); 
    // optional, handle chaining here, if value is another promise
  }

  reject(error) {
    if(this.status === 'resolved') return;
    this.status = 'rejected';
    release(error); 
  }
  
  release(value) {
    this.result = value; 
    this.callbacks.forEach((data) => {
      this.release(data.onResolved, data.onRejected);
    }, this)
  }
}