jQuery addClass example

Change class name on click in jQuery

HTML

<script src="https://rawgit.com/kangax/fabric.js/master/dist/fabric.js"></script>
 <button>Crop</button>
 <canvas id="canvas" width="500" height="500"></canvas>

JavaScript

// find elements
var button = $("button");

// handle click and add class
button.on("click", function(){
  canvas.renderAll();

  canvas.setDimensions({
    width: selectionRect.getScaledWidth(),
    height: selectionRect.getScaledHeight()
  })
  currentImage.set({
    left: -selectionRect.left,
    top: -selectionRect.top
  })
  canvas.remove(selectionRect);

  canvas.renderAll();
});

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

canvas.preserveObjectStacking = true;

var selectionRect;
var currentImage;

fabric.Image.fromURL('https://www.sheffield.ac.uk/polopoly_fs/1.512509!/image/DiamondBasement.jpg', img => {
  img.scaleToHeight(500);

  img.selectable = true;

  canvas.add(img);

  currentImage = img;
  
  canvas.backgroundColor = "#333";

  addSelectionRect();

  canvas.setActiveObject(selectionRect);

  canvas.renderAll();
});

function addSelectionRect() {
  selectionRect = new fabric.Rect({
    fill: 'rgba(0,0,0,0.3)',
    originX: 'left',
    originY: 'top',
    stroke: 'black',
    opacity: 1,
    width: currentImage.width,
    height: currentImage.height,
    hasRotatingPoint: false,
    transparentCorners: false,
    cornerColor: 'white',
    cornerStrokeColor: 'black',
    borderColor: 'black',
    cornerSize: 12,
    padding: 0,
    cornerStyle: 'circle',
    borderDashArray: [5, 5],
    borderScaleFactor: 1.3
  });

  selectionRect.scaleToWidth(300);
  canvas.centerObject(selectionRect);
  selectionRect.visible = true;
  canvas.add(selectionRect);
}