JavaScript: cannot access variable within nested objects

Question

So I am trying to create an object that holds another object inside of it when initialized. However, when I call a method in the outer object, it cannot find the inner object's variable.

class Game {

  constructor(){
    //creates canvas and ctx
    this.xPosition = 0
    this.yPosition = 0
    this.canvas = document.getElementById("canvas");
    this.canvas.style.background = "black";
    this.ctx = canvas.getContext("2d");
    this.drawSquare(0,0);
    this.snake = new Snake();
  }

  drawSquare(x,y){
    this.snake.head = [this.xPosition,this.yPosition];
    this.ctx.clearRect(this.xPosition, this.yPosition, 30, 30); //pop tail
    this.xPosition += x
    this.yPosition += y;
    if (this.xPosition < 0){
      this.xPosition = 600;
    }
    else if (this.xPosition > 600){
      this.xPosition = 0;
    }
    if (this.yPosition < 0){
      this.yPosition = 600;
    }
    else if (this.yPosition > 600){
      this.yPosition = 0;
    }
    this.ctx.fillStyle = "#FF0000";
    this.ctx.fillRect(this.xPosition,this.yPosition,30,30);
  }
}

class Snake {
  constructor(){
    this.head = [];
    this.tail = [];
    this.length = 1;
  }
}

When I run this code in the browser, I get an error: this.snake is undefined.

Before I even initialize an instance of Game, that is: var game = new Game(); I already get the error. — Fmak
– Fmak, Commented Jun 23, 2018 at 19:53
new Game() already throws an exception? what is the line of the code you are getting the exception? — Daniel Krom
– Daniel Krom, Commented Jun 23, 2018 at 19:56
the problem is with the line directly underneath the drawSquare method, this.snake.head = [this.xPosition,this.yPosition] — Fmak
– Fmak, Commented Jun 23, 2018 at 19:59

Daniel Krom · Accepted Answer · 2018-06-23 20:04:40Z

1

You are calling the method this.drawSquare that uses this.snake before initiating this.snake = new Snake()

Try replacing it on the constructor:

constructor(){
    //creates canvas and ctx
    this.xPosition = 0
    this.yPosition = 0
    this.canvas = document.getElementById("canvas");
    this.canvas.style.background = "black";
    this.ctx = canvas.getContext("2d");
    this.snake = new Snake();
    this.drawSquare(0,0); // this line changed
  }

answered Jun 23, 2018 at 20:04

Daniel Krom

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