FAQ

How can I reference existing objects on canvas?

canvas.getObjects() returns an array of all objects on canvas.
canvas.item(n) returns n-th object on canvas (where indexing is associated with z-order)

canvas.getObjects(); // [...]
canvas.item(0); // first item
canvas.item(1); // second item

How can I change object's properties?

Using fabric.Object#set method.

rect.set('width', 100).set('height', 50);
// or
rect.set({ width: 100, height: 50 })

How can I change background color of canvas area?

Using "backgroundColor" property of fabric.Element instance. E.g.: canvas.backgroundColor = 'red' or canvas.backgroundColor = rgba(0, 255, 0, 0.5)