Edit: This question and answer is much better and give a sense than the other answer that says 'duplicate'. Worth keeping this for futher refference, plus this is beginnner friendly as I have no clue what callbacl is and all of the confusing example of foo boo bar...

I get a value of null when calling a class inside a method of another class. I want to animate the object in JavaScript canvas. 

I'm making a small animation with JavaScrpt canvas and I'm also learning about classes and JavaScript in general. So a newbie question.

How can I remove the error and make this work?

Here is the codepen: <https://codepen.io/Aurelian/pen/mGWVbq?editors=0010>

Here is the JS:

```
'use strict';
function random(min, max) {
  return Math.random() * (max - min) + min;
}
   /*
    * ------------------------------------------
    * *-----------------------------
    *  User Event
    * *-----------------------------
    * ------------------------------------------
    */
   class UserEvent {
      constructor(canvasBody) {
         this.UP_ARROW = 38 || 87,
         this.RIGHT_ARROW =39 || 68,
         this.DOWN_ARROW = 37 || 83,
         this.LEFT_ARROW = 40 || 65,
         this.keys = []

         canvasBody.addEventListener('keydown', (e) => {
            this.keys[e.keyCode] = true;
         });

         canvasBody.addEventListener('keyup', (e) => {
            this.keys[e.keyCode] = false;
         });
      }

      checkKey(key) {
         return this.keys[key];
      }

//       ufoMove() {
//          this.canvasBody.checkKey(this.UP_ARROW)
//          this.canvasBody.checkKey(this.RIGHT_ARROW)
//          this.canvasBodycheckKey(this.DOWN_ARROW)
//          this.canvasBody.checkKey(this.LEFT_ARROW)
//          console.log(this.canvasBody.leftArrow = this.checkKey(this.LEFT_ARROW))

//          if(this.UP_ARROW) {
//             this.x += this.x * this.velocity.x
//          }
//       }
   }

   /*
    * ------------------------------------------
    * *-----------------------------
    *  UFO
    * *-----------------------------
    * ------------------------------------------
    */
   class Ufo {
      constructor(x, y) {
         this.x = x,
         this.y = y,
         this.velocity = {
            x: 3,
            y: 3
         }
      }

      draw(c) {
         c.save()
         c.beginPath()
         c.arc(this.x, this.y, 50, 0, Math.PI * 2, false)
         c.fillStyle = "#fff";
         c.shadowColor = "#e3eaef";
         c.shadowBlur = 20;
         c.fill()
         c.closePath()
         c.restore()
      }

      update(c) {
         this.draw(c)
         // Get the keys first
         // this.EventUser.ufoMove(c);
         this.x = this.x + random(-5,5);
         this.y = this.y + random(-5,5);
         //    this.x = this.x + this.velocity.x;
         // }
      }
   }

   /*
    * ------------------------------------------
    * *-----------------------------
    *  Canvas
    * *-----------------------------
    * ------------------------------------------
    */      
   class CanvasDisplay {
      constructor() {
         this.canvas = document.querySelector('canvas');
           this.ctx = this.canvas.getContext('2d');
         this.stageConfig = {
              width: window.innerWidth,
              height: window.innerHeight
         };         
         this.canvas.width = this.stageConfig.width;
         this.canvas.height = this.stageConfig.height;

         this.backgroundGradient = this.ctx.createLinearGradient(0, 0, 0, this.canvas.height);
         this.backgroundGradient.addColorStop(0, '#171e26');
         this.backgroundGradient.addColorStop(1, '#3f586b');

         this.Ufo = new Ufo(this.canvas.width / 2, this.canvas.height / 2);
         this.UserEvent = new UserEvent(document.body);
      }

      animate() {
         this.ctx.fillStyle = this.backgroundGradient;
         this.ctx.fillRect(0, 0, this.canvas.width, this.canvas.height);

         this.Ufo.update(this.ctx)
         window.requestAnimationFrame(this.animate);
      }
   }

   let canvasDisplay = new CanvasDisplay();
   canvasDisplay.animate();

```