How can I get info such as symbol of an ERC20 token on RSK?

To do this using web3.js:

• web3: Have a web3 instance initialised and connected to a web3 provider
• abiOfToken: The ABI object for this particular token. Note that if you do not have this, you can obtain it by running solc against the original contract code; or alternatively you can simply use a "standard" ABI object for ERC-20 tokens
• addressOfToken: The deployed smart contract address for the token

Once you have the above, you can do the following within an async function:

  const tokenContract = new web3.eth.Contract(
    abiOfToken, addressOfToken);
  const symbol = await tokenContract.methods.symbol().call();
  const decimals = await tokenContract.methods.decimals().call();
  const name = await tkenContract.methods.name().call();

The above code runs them in sequence, and provided for clarity. In practice, since you are running this for multiple tokens, you may want to consider running the queries in parallel, and extracting them to a separate function, like so:

  // run this just once, as part of initialisation
  const tokenContract = new web3.eth.Contract(abiOfToken, addressOfToken);

  // run this multiple times by putting in its own function
  async function getTokenInfo(tokenContract) {
    const [decimals, name, symbol] = await Promise.all([
      tokenContract.methods.symbol().call(),
      tokenContract.methods.decimals().call(),
      tokenContract.methods.name().call(),
    ]);
    return { decimals, name, symbol };
  }