JavaScript

Object.prototype.toString() - JavaScript | MDN[^]:

For Numbers and BigInts toString() takes an optional parameter radix the value of radix must be minimum 2 and maximum 36.

By using radix you can also convert base 10 numbers (like 1,2,3,4,5,.........) to another base numbers, in example below we are converting base 10 number to a base 2 (binary) number ...

value = alphabet.indexOf(text[letter].toUpperCase()) + 1;
binary += value.toString(2).padStart(5, "0");

const searchParams = new URLSearchParams(location.search);
for (const [key, value] of searchParams) {
    console.log(key, "=", value);
}

const searchParams = new URLSearchParams(location.search);
for (const [key, value] of searchParams) {
    let element = document.getElementsByName(key)[0];
    if (element && element.classList.contains("dropdown") && (/^INPUT|SELECT$/i).test(element.tagName)) {
        element.value = value;
    }
    else {
        console.warn("Unknown element:", key, element);
    }
}

const searchParams = new URLSearchParams(location.search);
let index = 0;

for (const [key, value] of searchParams) {
    let element = document.getElementsByClassName("dropdown")[index];
    element.value = value;
    index++;
}

let element = document.getElementsByClassName("dropdown")[index];
let option = document.createElement("option");
option.value = value;
option.text = value;
element.add(option, null);
element.value = value;

 <div class='payroll-container row p-5'>	
<form action ="overheads5.html"  method 
= "get" name = "payrate" id = 
"payrate"> Payrate
<br/>

Level 1

<input type = "text" name = 
"level-1" id = "level-1" />
<br/>

Level 2
<input type = "text" name = 
"level-2" id = "level-2" />
<br/>

Level 3
<input type = "text" name = 
"level-3" id = "level-3" />
<br/>

Level 4
<input type = "text" name = 
"level-4" id = "level-4" />
<br/>

Level 5
<input type = "text" name = 
"level-5" id = "level-5" />
<br/>

Level 6
<input type = "text" name = 
"level-6" id = "level-6" />
<br/>
<input type = "submit"/>

<div class='dropdown-container row p-4'>	  
<select>
<option class = 'dropdown'>level-1</option>
<option class = 'dropdown'>level-2</option>
<option class = 'dropdown'>level-3</option>
<option class = 'dropdown'>level-4</option>
<option class = 'dropdown'>level-5</option>
<option class = 'dropdown'>level-6</option>
	  </select></div>

const searchParams = new URLSearchParams(location.search);
for (const [key, value] of searchParams) {
    let element = document.getElementById(key);
    if (element) {
        element.value = value;
    }
    else {
        console.warn("Unknown element:", key, element);
    }
}

<script>
const searchParams = new URLSearchParams(location.search);
for (const [key, value] of searchParams) {
    let element = document.getElementById(key);
    if (element) {
        element.value = value;
    }
    else {
        console.warn("Unknown element:", key, element);
    }
}
</script>

<div class='dropdown-container row p-4'>
    <select>
        <option class='dropdown'>level-1</option>
        <option class='dropdown'>level-2</option>
        <option class='dropdown'>level-3</option>
        <option class='dropdown'>level-4</option>
        <option class='dropdown'>level-5</option>
        <option class='dropdown'>level-6</option>
    </select>
</div>

const searchParams = new URLSearchParams(location.search);
const elements = document.getElementsByClassName("dropdown");
let index = 0;

for (const [key, value] of searchParams) {
    let element = elements[index];
    element.text = value;
    index++;
}