Z- Axis - Fabric.js

HTML

<canvas id="c" width="500" height="500"></canvas>

JavaScript

var canvas  = new fabric.Canvas('c');

var pol = new fabric.Polygon([
  {x: 200, y: 0},
  {x: 250, y: 50},
  {x: 250, y: 100},
  {x: 150, y: 100},
  {x: 150, y: 50} ], {
    left: 250,
    top: 150,
    angle: 0,
    fill: 'green'
  }
);

var pol2 = new fabric.Polygon([
  {x: 200, y: 50},
  {x: 200, y: 100},
  {x: 100, y: 100},
  {x: 100, y: 50} ], {
    left: 300,
    top: 200,
    angle: 0,
    fill: 'blue'
  }
);
canvas.add(pol, pol2);

canvas.on('object:selected', function(event) {
        var object = event.target;
        canvas.sendToBack(object);
        //object.sendToBack();
        console.log("OK");
  });