The checksum.js file contains genchecksum() and verifychecksum() functions. The merchant module should call these methods with the appropriate set of parameters as mentioned in the API document given here
Keep all the files in the folder from where you will be calling the genchecksum() and verifychecksum() methods.
The checksum/server.js file contains set required parameters and code to generate checksum to run the default transaction flow.
For Offline(Wallet Api) Checksum Utility below are the methods:
genchecksumbystring : For generating the checksum
verifychecksumbystring : For verifing the checksum