if(value.includes('your string')) {
    buttonElement.disabled = true
} else {
    buttonElement.disabled = false
}

If you use this pattern in onChange event it would always be updated based on the value

const buttonElement = document.querySelector('button')
const inputElement = document.querySelector('input')

inputElement.addEventListener('change', (e) => {
    if(e.target.value.includes('old.company.com')){
        buttonElement.disabled = false
    } else {
        buttonElement.disabled = true
    }
}

This is how I validate form fields if its not too complicated,
you could also use regex match with this pattern.

Maybe you can use the “onchange” attribute on the input.

It will give you the value of the input everytime that the element is updated

https://www.w3schools.com/jsref/event_onchange.asp

HTML

<input onchange="handleChange(this" />

JS => You’ll get a string

const button = document.getElementById('myButton')

function handleChange(event){
 let inputValue = event.value;
 console.log(inputValue)


}

Now you must compare the inputValue

If the inputValue is not equal to your domain => button.setAttribute('disabled','') // Means that we add the disabled attribute to the button

And in the case if your domain is OK then => button.removeAttribute('disabled')

In your code :

new URL("https://www.old.company.com/products").hostname === 'www.old.company.com'

function handleChange(event){
 const urlInput = event.target.value.startswith('https://') ? 
 event.target.value : `https://${event.target.value}`;

 if(new URL(urlInput).hostname === 'www.old.company.com')
   return document.querySelector('.button').disabled = false;
 return document.querySelector('.button').disabled = true;
}

I have simple app that has one input (URL) and one button.

The button has some JavaScript that takes the URL and replaces the domain with a new domain.

Basically, a user enters “www.old.company.com/products” and the app returns “www.new.company.com/products”.

The program works, but now I’m trying to make it so the button is disabled until the input has a specific domain. I tried adding a disabled attribute to the button, but I can’t figure out how to get remove the attribute based on the input value.

For example:

• www.old.company.com/products > button enabled
• www.old.company.com/about-us > button enabled
• www.companyX.com/products > button disabled
• www.google.com > button disabled
• blank > button disabled

function replace() {
  var a = document.getElementById('input').value;
  var b = a.replace(/old.company.com/g, "new.company.com");
  document.getElementById('output').innerHTML = b;
}

button {
  background-color: #003359;
  min-width: 150px;
  border: none;
  color: #ffffff;
  padding: 10px 20px;
  text-align: center;
  text-decoration: none;
  font-size: 14px;
  text-transform: uppercase;
  font-weight: bold;
  margin: 0px 20px 20px 0px;
  border-radius: 2px;
}

button:disabled {
  background-color: #f5f5f5;
  min-width: 150px;
  border: none;
  color: #9A9A9A;
  padding: 10px 20px;
  text-align: center;
  text-decoration: none;
  font-size: 14px;
  text-transform: uppercase;
  font-weight: bold;
  margin: 0px 20px 20px 0px;
  border-radius: 2px;
}

<div>
  <input id="input" style="width: 30%; margin: 20px;" type="text" placeholder="https://old.company.com/..." pattern="^(http://www.|https://www.|http://|https://)?old+([-.]company+).[a-z]{2,5}(:[0-9]{1,5})?(/.*)?$" />
  <button onclick="replace()" class="button">Create</button>
  <p id="output">&nbsp;</p>
</div>

https://jsfiddle.net/ianjmonk/637quxLn/9/