Your problem is related to the asynchronous nature of the JavaScript execution: the code located in the pool.query's callback function runs after the assignation of the h variable and after the console.log.

You have 2 options:

1. You can place your entire follow up code inside the callback function:

pool.query("select * from tblusers;", (err, res) => {
  const h = res
  // do whatever comes next here...
})

2. Promisify the result of the callback function and await its resolution before moving on

const h = await new Promise((resolve) => {
  pool.query("select * from tblusers;", (err, res) => {
    resolve(res)
  })
})

console.log(h)
// do whatever comes next here...

Note that if you are not using one of the latest versions of node (supporting top level await), you will have to place everything in an IIFE function:

(async () => {
  const pool = createPool({
    host:"localhost",
    user:"root",
    password:"",
    database:"ticketsystem",
    connectionLimit:10
  })

  const h = await new Promise((resolve) => {
    pool.query("select * from tblusers;", (err, res) => {
      resolve(res)
    })
  })

  console.log(h)
  // do whatever comes next here...
})()