[Ethereum] Why do we need to specify constant in the return value

Which version of web3 are you using?

Here is a working example with web3@^1.0.0-beta.34

const Web3 = require('web3');
contractAddress = '0xe7775a6e9bcf904eb39da2b68c5efb4f9360e08c';
provider = 'https://mainnet.infura.io';
const contractAbiJSON = '';//... The JSON abi

// Init
const web3 = new Web3(new web3.providers.HttpProvider(provider));

const contract = new web3.eth.Contract(contractAbiJSON, contractAddress);

contract.methods.totalSupply().call().then((totalSupply) => {
  console.log(totalSupply);
});

Well you say it yourself, you're not calling aString, but super.printMe().

Have a deeper explanation of superhere:

The super keyword in Solidity gives access to the immediate parent contract from which the current contract is derived.

You are explicitly calling the parent contract Hello, in which aString is 'Hello, world.'

Look at the difference if in printMe2() you put:

return super.aString;