Using "if/else" with OnClick

Question

First of all, is it even possible to write if/else statements directly in html with onclick attribute? And if so, why is my code not working?

So this is a button. The "Calc.Input.value" refers to a text input and I want to display an error message if that field is 0 or blank. In all other cases I want some other things to happen as you can see. This is not relevant though, as everything after the "else" worked fine before.

<INPUT TYPE="button" NAME="drop" id="dropbutton" style="background-color:#D1E8D5;" VALUE="
Släpp ankare " OnClick="if ("Calc.Input.value == ''" || "Calc.Input.value == '0'")
{window.alert("Please enter a number");} else 
{document.getElementById('dropbutton').value=' Justera 
längd ';document.getElementById('length').innerHTML =
Calc.Input.value;document.getElementById('dropbutton').style.cssText = 'background-
color:#FFF6B3;';}">

Answer 1

Just DON'T put it in the onclick attribute.

Use

<INPUT TYPE="button" NAME="drop" id="dropbutton" style="background-color:#D1E8D5;" VALUE="Släpp ankare ">
<script>
document.getElementById('dropbutton').onclick = function() {
    if (Calc.Input.value == '' || Calc.Input.value == '0') {
         window.alert("Please enter a number");
    } else {
        document.getElementById('dropbutton').value=' Justera längd ';
        document.getElementById('length').innerHTML = Calc.Input.value;
        document.getElementById('dropbutton').style.backgroundColor = '#FFF6B3';
    }
    return false;
}
</script>