Subscribe via json-rpc has no response

Question

I don't get any response from json-rpc subscription.

It seems I need a type paramater, but the provider.subscribe() method does not document the type parameter. (I checked the js, java and python code, can't find any explanation of this param).

const { WsProvider } = require('@polkadot/api');

(async () => {

  const provider = new WsProvider('wss://kusama-rpc.polkadot.io');
  var count = 0

  const type = '' // @param string <==== what should this be?
  const method = 'chain_subscribeAllHeads'
  // const method = 'subscribeAllHeads' // => method not found
  // const method = 'state_subscribeRuntimeVersion'
  const params = []

  const unsub = await provider.subscribe(type, method, params, (err, response) => {
    count++
    console.log(err, response)
    if(count > 2) {
      unsub()
      process.exit(0)
    }
  })
})()