All that is required is a COBOL copybook. The COBOL must be properly formatted and have one, and only one, 01 level wrappering the entire data structure.
To generate the JavaScript object go to https://tools.hostbridge.com/copybook, and follow the instructions on the page.
The following page allows you to generate a JavaScript object that describes a 3270 screen from captures performed using the HostBridge Application Explorer. Creating web services using HostBridge does not require the use of these screen objects, but they can provide a layer of abstraction between the web services you create and the 3270 screens they interact with. The object can provide consistent field names and transformations into and out of the screen. If the screen should change only the object must be updated, not all of the services that use the object.
To use this page just capture the screens you wish to have JavaScript objects generated for using the HostBridge Application Explorer. Rename the capture file to *.zip and unzip it. Select the options for the generated code and upload the unzipped .hsg screen files.
To generate a JavaScript screen object go to https://tools.hostbridge.com/fieldgen, and follow the instructions on the page.