Promise return Promise status pending and promise value undefined in Odoo js

Question

Hello I am trying to convert string html to image using html2canvas library in my Odoo POS Module v8. But, when I try to use this function

var receipt_data = html2canvas($(receipt)).then(function(canvas) {
            $('body').appendChild(canvas);
        });

it gives -

Promise {[[PromiseStatus]]: "pending", [[PromiseValue]]: undefined}

in receipt_data I have included html2canvas.js in my assets module. Please help me...