How to keep the image at the back of the HTML5 canvas when moving elements with fabric.js?

To keep the image at the back of the canvas, when move elements, you need to pass:

preserveObjectStacking

And the following would work and the image is not visible in the background:

window.canvas = new fabric.Canvas('c', { preserveObjectStacking:true });

Now, on moving the shape will appear over the image.