Bind with anonymous function. Source: http://dailyjs.com/2012/06/25/this-binding/

bind.js

Shape.prototype = {
  move: function(x, y) {
    this.x += x;
    this.y += y;

    var checkBounds = function(min, max) {
      if (this.x < min || this.x > max) {
        console.error('Warning: Shape out of bounds');
      }
    }.bind(this);

    checkBounds(0, 100);
  }
};