form submit override with javascript

HTML

<form id="myform" onsubmit="event.preventDefault(); validate_form();">
	<input type="text" placeholder="My Awesome Form Element">
  	<input type="Submit" value="Submit Me!">
</form>

Javascript

function validate_form()
{
  var valid = false; // check form elements here and then set to true if they are valid
  if(valid == false) { 
    alert("validation failed false");
    returnToPreviousPage();
    return false;
  }

  alert("validations passed");
  return true;
}

// form validation