eth_getStorageAt

storage.js

var Web3 = require('web3');
var web3 = new Web3();

var key = Buffer.from("000000000000000000000000391694e7e0b0cce554cb130d723a9d27458f9298", "hex");
var pos = Buffer.from("0000000000000000000000000000000000000000000000000000000000000001", "hex");

var key = Buffer.concat([key, pos], key.length + pos.length).toString("hex");

var options = {encoding: "hex"};
var position = web3.sha3(key, options);

console.log("position", position);