Simple JavaScript Checkbox Validation

Question

I usually work with PHP so sadly don\'t have some basic JS principles down. This is all I want to accomplish--I\'ve seen many posts on this topic but they are usually beyon

Answer 1

var testCheckbox = document.getElementById("checkbox");  
if (!testCheckbox.checked) {
  alert("Error Message!!");
}
else {
  alert("Success Message!!");
}