I'm taking address and viewkey as input then want to return its balance in my expressjs project. How to call monero blockchain from nodejs for checking balance of any paper wallet?
There are several nodejs packages for working with wallets. They essentially just wrap the JSON-RPC methods on the daemon / wallet RPC interfaces.
You don't actually need a wrapper though as you can just call the JSON-RPC methods directly. For getting the balance, you would call this. The examples use cURL at the command line, but if you know how to make HTTP requests in nodejs, it should be pretty obvious what request payloads to use and the example responses show what JSON data you'll get back.