function check(){
 	for(i=0; i<=4; i++){
	inp = document.forms[0].elements[i];
		if(!inp.value){
		alert('All fields are required.');
		return false;}
	}
return true;
}

function form_set(){	
document.forms[0].onsubmit = check;		//sets form eventhandler onsubmit. 
}

window.onload = form_set;