I am providing a form where the user shall enter an arithmetic calculation. Further down the result shall appear, once the user hits enter. It might just be a problem of syntax, but I couldn't find the mistake. Here is what I did so far:

```html
<!DOCTYPE html>
<html>
<body>
<p>What do you want to calculate?</p>
<form method="post"><span>Type here:</span><input type="text" id="calc"></input>
</form>
<script>
num_field = document.getElementById("calc");
num_field.onsubmit=function ()
{
    document.getElementById("display_result").innerHTML = num_field;
}
</script>
<p id="display_result"></p>
</body>
</html>

```

So, the user shall enter for instance "1+2". The result shall appear below.
Any idea where is my mistake?

Best regards