Pixel List defines a custom iterator to iterate the list of Pixel containing x and y coordinated in a sorted order based on their distance from (0, 0). Based on the idea in Getify's YDKS This and Object Prototypes Book to define a custom iterator

pixel-list.js

const Pixel = require('./pixel-it');

// Contains all the pixels within a list
const pixelList = []

// Custom iterator of object is accessed by using 
// Symbol.iterator a computed property name
pixelList[Symbol.iterator] = function () {
  // Defining a function for the custom iterator
  const sortedList = sortList(this);
  let it = 0;

  return {
    next: function () {
      return {
        value: sortedList[it++],
        done: it > sortedList.length
      }
    }
  }
}

const clone = a => a.map(v => v)

function sortList (pixelList) {
  const clonedList = clone(pixelList);
  return clonedList.sort((pixel1, pixel2) => {
    const distance1 = distZero(pixel1);
    const distance2 = distZero(pixel2);

    if (distance1 > distance2) return 1;
    else if (distance1 < distance2) return -1;
    else if (distance1 === distance2) return 0;
  })
}

function calcDist (pixel2, pixel1) {
  return Math.sqrt(Math.pow(pixel2.x - pixel1.x, 2) + Math.pow(pixel2.y - pixel1.y, 2))
}


distZero = calcDist.bind({}, new Pixel(0, 0));

for (var i = 0; i < 50; i++) {
  for (var j = 0; j < 50; j++) {
    pixelList.push(new Pixel(i, j));
  }
}

// This will iterate the pixel in a sorted order based on the function 
// distZero to calculate the distance of all the pixels from point (0, 0) on the screen. 
for (var pixel of pixelList) {
  console.log(distZero(pixel));
}