WHY CAN'T THIS CODE WORK

Question

var maths;
//math section
maths=(prompt("Enter Your Maths score"));

if (maths=== "" )
alert("PLEASE FILL OUT BEFORE YOU SUBMIT THE FORM")
(prompt("Enter Your Maths score")),
else
document.write("THANKS FOR YOUR TIME "+ "<br />" );

I WANT TO ALERT WHEN THE USER TRY TO SUBMIT AN EMPTY FORM, LIKE
alert("PLEASE FILL OUT BEFORE YOU SUBMIT THE FORM")
AND ALSO I WANT THE CODE TO TAKE USER BACK TO SUBMIT A FORM LIKE
(prompt("Enter Your Maths Marks")),
BUT IT DOESN'T WORK I DON'T KNOW WHY ACTUALLY

Answer 1

Cause of the lack of { } ; ?
+ You should use a do while boucle here

Maths="";
Maths = prompt("fill out math score");
Do{
Maths=prompt("please fill out math score");
}while(Maths=="" || isNaN(Maths));

isNaN verify is not a number

Answer 2

Curly braces are compulsory in Javascript.
if(condition){
statement
}else{
statement
}
Something like this

Answer 3

Try to check Maths length.