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<!DOCTYPE html>
<html>
<head>
<meta name="description" content="Promise Shift Gears">
  <meta charset="utf-8">
  <meta name="viewport" content="width=device-width">
  <title>Promise Shift Gears</title>
</head>
<body>
​
</body>
</html>
 
const Bike = function(frontIndex, rearIndex){
  
  this.frontGearIndex = frontIndex || 0;
  this.rearGearIndex = rearIndex || 0; 
  
  this.transmission = {
    frontGearTeeth: [30,45],
    rearGearTeeth: [11,13,15,17,19,21,24,28,32,36]
​
  }
}
​
Bike.prototype.calculateGearRatio = function(){
  let front = this.transmission.frontGearTeeth[this.frontGearIndex],
      rear = this.transmission.rearGearTeeth[this.rearGearIndex];
  
  console.log(`front: ${front}, rear: ${rear}`);
  
  if (front && rear) {
    return (front / rear) ;
  } else {
    return 0; 
  }
  
};
​
Bike.prototype.changeBothGears = function(frontChange, rearChange) {
  
  let shiftFront = {
    currentIndex: this.frontGearIndex,
    maxIndex: this.transmission.frontGearTeeth.length - 1,
    changeBy: frontChange
  };
  
  let shiftRear = {
    currentIndex: this.rearGearIndex,
    maxIndex: this.transmission.rearGearTeeth.length - 1,
    changeBy: rearChange
  }
  
  this.changeGearAsync(shiftFront)
    .then(
      (newIndex) => {
        this.frontGearIndex = newIndex;
      
        console.log(this.calculateGearRatio());
        
        return this.changeGearAsync(shiftRear);
      }
    )
    .then(
      (newIndex) => {
        this.rearGearIndex = newIndex;
      
        console.log(this.calculateGearRatio());
​
      }
    )
    .catch(
      (err) => {console.log("Error: " + err);}
    );
​
  
};
​
Bike.prototype.changeGear = function(frontOrRear, changeBy) { 
  
  let shiftIndexName = frontOrRear + "GearIndex"
  
  //contains state change for making the shift
  let shiftObject = {
    currentIndex: this[shiftIndexName], 
    maxIndex: this.transmission[frontOrRear + "GearTeeth"].length -1,
    changeBy: changeBy
  }
  
  // invoke async function that returns a promise
  this.changeGearAsync(shiftObject)
    .then(
      (newIndex) => {
​
        this[shiftIndexName] = newIndex;
        
        console.log(this.calculateGearRatio());
        
      }
    )
    .catch(
      (err) => {console.log("Error: " + err);}
    );
 };
​
/**
* take in shiftObject, create new gear state and pass back through callback
* if error, invoke callback with error
*/
Bike.prototype.changeGearAsync = function(shiftObject){
  
  return new Promise(
    (resolve, reject) => {
      
      let newIndex = shiftObject.currentIndex + shiftObject.changeBy; 
​
      if (newIndex < 0 || newIndex > shiftObject.maxIndex) {
        reject("New Index is Invalid: " + newIndex);
      } else {
        resolve(newIndex);
      }
​
    }
  );
};
​
const bike = new Bike(1,8);
console.log(bike.calculateGearRatio()); // 1.40625
bike.changeGear("front", -1);           // 0.9375
bike.changeGear("rear", 1);             // 0.8333...
bike.changeGear("front", -1);           // lower than zero so no change
​
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