Trying to call a js function inside PHP code

Question

I'm running a PHP code to validate login form and if there's an input empty it should run a javscript function that changes the display attribute of some DIVs to block. If I put my JS code between the head tag it says

Uncaught TypeError: Cannot read property 'style' of null

, and If I put my code before the /body tag it says

Uncaught ReferenceError: formValidation is not defined

Javascript code:

function formValidation() {
     document.getElementById("errMessage").style.display = "block",
    document.getElementById("arrow-errMessage").style.display = "block",
    document.getElementById("errEmail").style.display = "block",
    document.getElementById("arrow-errEmail").style.display = "block"; 
} 

PHP code:

else {
          echo "<script> formValidation(); </script>";
    }

Answer 1

At the time you execute formValidation() the DOM elements are not ready.
You need to catch the page load event and then execute the style change.

document.addEventListener("load", formValidation);

Answer 2

Try to use

window.onload=formValidation; 

It is executed when web page has completely loaded all content.

Answer 3

You Can use JQuery for checking document ready event.

 <script type="text/javascript" src="https://code.jquery.com/jquery-1.12.3.min.js"></script>

Use this HTML code in your document

else{
echo '<script type="text/javascript">
    $( document ).ready(function() {
       formValidation();
    });
</script>';
}

Edit your php code and try this