Low-level integer numbers reversing:

function flipInt(n){
    var digit, result = 0

    while( n ){
        digit = n % 10  //  Get right-most digit. Ex. 123/10 → 12.3 → 3
        result = (result * 10) + digit  //  Ex. 123 → 1230 + 4 → 1234
        n = n/10|0  //  Remove right-most digit. Ex. 123 → 12.3 → 12
    }  
  
    return result
}


// Usage: 
alert(
  "Reversed number: " + flipInt( +prompt("Enter a value") ) 
)

The above code uses bitwise operators for quick math

This method is MUCH FASTER than other methods which convert the number to an Array and then reverse it and join it again. This is a low-level blazing-fast solution.

Illustration table:

Show code snippet

const delay = (ms = 1000) => new Promise(res => setTimeout(res, ms))
const table = document.querySelector('tbody')

async function printLine(s1, s2, op){
  table.innerHTML += `<tr>
    <td>{s2||''}</td>
  </tr>`
}

async function steps(){
  printLine(123)
  await delay()
  
  printLine('12.3 →')
  await delay()
  
  printLine(12, 3)
  await delay()
  
  printLine('1.2', '3 &times; 10')
  await delay()
  
  printLine('1.2 →', 30)
  await delay()
  
  printLine(1, 32)
  await delay()
  
  printLine(1, '32 &times; 10')
  await delay()
  
  printLine('1 →', 320)
  await delay()
  
  printLine('', 321)
  await delay()
}

steps()

table{ width: 200px; }
td {
  border: 1px dotted #999;
}

<table>
  <thead>
    <tr>
      <th>Current</th>
      <th>Output</th>
    </tr>
  </thead>
  <tbody>
  </tbody>
</table>

You do not return the result. Use

return num.split("").reverse().join("");

If you use just return, then undefined is returned.

function reverse_the_string(num) {
    return String(num).split("").reverse().join("");
}

document.write(reverse_the_string(56789));

Caveat: Reversing works only with small numbers!