Unsupported non-pure initializer in .\index.js on line 1

I'm trying to use a .JS file in unity and it contains websockets implementation. and the way to get websockets is:

var WebSocket = require('ws');

But this line causes the Liberary build to fail and provides following errors:

Unsupported non-pure initializer in .\index.js on line 1
Initialize variables in a function and invoke it from inside main()

I'm new to JS and I dont know how to invoke it inside main(). Can you please help me.