Override fabric Object to add custom attribute

fabric-toObject.js

//when you export canvas.toJSON(), the object will cointain `your_custom_attribute`
//{“objects”: [{“src”: “”, “your_custom_attribute”: “”, “type”: “image”, …}, {}], “version”: “2.0.0-rc.4” }

fabric.Object.prototype.toObject = (function (toObject) {
    return function (propertiesToInclude) {
        propertiesToInclude = (propertiesToInclude || []).concat(
            ['your_custom_attribute', ...]
        );
        return toObject.apply(this, [propertiesToInclude]);
    };
})(fabric.Object.prototype.toObject);