why does it not work

Hi Mully,

Changing the href attribute of a button doesn't make sense since buttons don't have href attributes. If you want to open the mail client after filling the email prompt you could use:

<button onclick="myFunction();" id="myd">save</button>

<script> 
  function myFunction(){ 
    var email = prompt("whats your email"); 
    window.location.href = "mailto:" + email + "?subject=Cabbages"; 
  } 
</script>

I am not sure what your function is trying to do that it is not working, I ran your code with an additional function as a callback to check its functionality:

function myFunction(checkHref) { 
  var email = prompt("whats your email"); 
  document.getElementById("myd").href = "mailto:" + email + "?subject=Cabbages"; 
  checkHref();
} 

checkHref = () => {
  console.log(document.getElementById("myd").href);
}

On clicking the button, the href attribute is maybe updating too late for what you need. If you are using this data or transferring the value synchronously, the href value is not updating before you use it. In my code above, the checkHref() function is running before the href is updated which will cause the console to log whatever the last value for href was of "myd".

To fix this, a delay needs to exist between when it's set and when it's used.