ReferenceError: Epson is not defined while running ePosDev = new Epson.EPOSDevice();

We want to implement receipt printing functionality, for which we are using epson sdk(javascript based) so we placed epos-2.13.0.js in our root folder of the server and then in our html page, we just used var ePosDev = new Epson.EPOSDevice();

but while it executes new Epson.EPOSDevice(), it encounters "ReferenceError: Epson is not defined" Can somebody guide me what is going wrong here.