title: ngOnChanges tracking previous value and new value?
body: <p>In my angular project, I'm trying to be able to go through a list by hitting next/previous buttons. I have a checkbox that changes the hours displayed from hours to days. I want to be able to go back and forth through the employees and have the values be correct. I'm told that this can be done through ngOnChanges and tracking the previous and new value.</p>
  
  <p>This is what I have, which is changing the new value, but how would I change the previous value?</p>
  
  <pre><code> ngOnChanges(changes: { [propName: string]: SimpleChange }) {
      for (let propName in changes) {
          if (propName == "selectedEmployee") {
              if (this.timeVar == "days") {
                  this.empInfo[this.selectedEmployee].STDLTD = this.empInfo[this.selectedEmployee].STDLTD * 8;
                  this.empInfo[this.selectedEmployee].Uncharged = this.empInfo[this.selectedEmployee].Uncharged * 8;
  
                  this.empInfo[this.selectedEmployee].PTOBase = this.empInfo[this.selectedEmployee].PTOBase * 8;
                  this.empInfo[this.selectedEmployee].PTOCarry = this.empInfo[this.selectedEmployee].PTOCarry * 8;
                  this.empInfo[this.selectedEmployee].PTOBorrowed = this.empInfo[this.selectedEmployee].PTOBorrowed * 8;
                  this.empInfo[this.selectedEmployee].PTOBalance = this.empInfo[this.selectedEmployee].PTOBalance * 8;
                  this.empInfo[this.selectedEmployee].PTORequests = this.empInfo[this.selectedEmployee].PTORequests * 8;
                  this.empInfo[this.selectedEmployee].PTORemaining = this.empInfo[this.selectedEmployee].PTORemaining * 8;
  
                  this.empInfo[this.selectedEmployee].ETOEarned = this.empInfo[this.selectedEmployee].ETOEarned * 8;
                  this.empInfo[this.selectedEmployee].ETORequests = this.empInfo[this.selectedEmployee].ETORequests * 8;
                  this.empInfo[this.selectedEmployee].ETORemaining = this.empInfo[this.selectedEmployee].ETORemaining * 8;  
              }
          }
      }
  }
  </code></pre>