Javascript element is null [duplicate]

Answer 1

Put this after or below input element. The input element wasn't loaded yet and the javascript is executed so you get null.

Solution 1:

Solution 2:

Or if you preffer putting scripts in the head of html, run the function only when the page is loaded.

On your separate javascript page:

function show()
{
    var supporter = document.getElementById('supporter');
     if (supporter.value == "yes") 
     {
       alert("it works");
     }
}

//When page loaded
document.onload = function(){
    show();
};

Note: you don't have to put in your JavaScript file.