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Editorial Team
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Asked: 2026-05-20T23:33:36+00:00

Today is my first day in JavaScript. The book (JavaScript Definitive Guide) has an

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Today is my first day in JavaScript. The book (JavaScript Definitive Guide) has an excersice printing all the factorials.

This is what I did:

<html>
<b><head> Factorial - JavaScript - Ex1</head></b>
<body>
        <h2> Factorials List </h2>
<script>

        var fact = 1; 
        var num = 1;
        for(num <= 10; num++)
            {
                fact=fact*num;
                document.write(num + "! = " + fact + "<br>");

            }
</script>
</body>
</html>

There is a problem which I don’t exactly know. I checked the book and the way that writer solved it was by initializing the variable num inside the loop FOR. I did that and it worked.
But what is the difference between that and mine?

Enlighten me Experts 🙂

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1 Answer

  1. Editorial Team

    A for loop‘s syntax must be

    for (<initializer>; <condition>; <increment>) {
    <body>
}

    While any of <initializer>, <condition> and <increment> can be omitted, none of the semicolons ; can be removed. That means, your for loop must be written with an extra semicolon:

    var num = 1;
for(; num <= 10; num++)
//  ^

    Or just move the var num = 1; into the for, which is normally what people would do:

    for (var num = 1; num <= 10; num ++) 
//   ^^^^^^^^^^^^
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