проверка

js/script.js

var Obj = {
val: 10,
str: 'Hello World!'
}
delete Obj.val;
function funMake() {
if('val' in Obj) {
document.write(Obj.val + '<br />');
}
else {
document.write('Error<br />');
}
if('str' in Obj) {
document.write(Obj.str + '<br />');
}
else {
document.write('Error<br />');
}
if('a' in Obj) {
document.write(Obj.a + '<br />');
}
else {
document.write('Error<br />');
}
}

var Obj = {
 val: 10, 
 str: 'Hello World!'
}

delete Obj.val;

function funMake() {
 if('val' in Obj) {
  document.write(Obj.val + '
');
 }
 else {
  document.write('Error
');
 }
 if('str' in Obj) {
  document.write(Obj.str + '
');
 }
 else {
  document.write('Error
');
 }
 if('a' in Obj) {
  document.write(Obj.a + '
');
 }
 else {
  document.write('Error
');
 }
}

index.html

<!DOCTYPE html>
<html>
<head>
<meta charset="utf-8" />
<title>New Page</title>
<script src="js/script.js"></script>
</head>
<body>
<script>
funMake();
</script>
</body>
</html>

<!DOCTYPE html>
<html>
<head>
<meta charset="utf-8" />
<title>New Page</title>
<script src="js/script.js"></script>
</head>
<body>
<script>
funMake();
</script>
</body>
</html>