difference of use comma or semicomma

Question

I have question concerning the difference of using in JavaScript code comma OR semicolon.

Do they have differences and influence how the code works?

In the code below I changed the semicolon after definition of var fib that have value 0 by comma an code didn't run.

Can somebody explain this?

The full code of Fibonacci in which it has occurred is here:

<script>
    document.write("<h2>Числа Фибоначчи </h2>");
    for (i = 0, j = 1, k = 0, fib = 0; i < 50; i++, fib = j + k, j = k, k = fib) {
        document.write("Fibonacci (" + i + ") = " + fib);
        document.write("<br>");
    }
</script>

This is a typical syntax of `for`-loop statements in JavaScript. — VisioN, Mar 19 '13 at 20:39
@gdoron You don't think that `var a = 1, b = 2;` is typical for javascript? At least very common. — dfsq, Mar 19 '13 at 20:43
Your code should be working, see http://jsfiddle.net/FfKRv/. Are you getting any errors in the console? — bfavaretto, Mar 19 '13 at 20:47
@dfsq common for sure. I use this all the time. it saves space — iAmClownShoe, Mar 19 '13 at 20:47

score 0 · Answer 1 · answered Mar 19 '13 at 20:39

0

for encloses 3 expressions :

the initialization (run before the loop),
the end of loop test (run before each iteration),
and the after iteration statement.

Statements in javascript are separated by ;. If you change the ; to a ,, you don't end the init statement.

answered Mar 19 '13 at 20:39

drunken bot

486
3
8

1

Commas separate arguments to a statement. Semicolons separate statements – HairOfTheDog Mar 19 '13 at 20:41

Jan Groth · Answer 2 · 2013-03-19T21:35:44.483

The part that probably confuses you is the fact that a 'normal' for-loop takes three statements like the following:

for (var i=0; i<5; i++)
  {
  x=x + "The number is " + i + "<br>";
  }

but that your example substitutes these expressions with comma seperated arguments.

var i=0 -> i = 0, j = 1, k = 0, fib = 0

i++ -> i++, fib = j + k, j = k, k = fib

BTW, this is terrible code that reads very hard and is even harder to maintain. Don't use it.

score 0 · Answer 3 · answered Mar 19 '13 at 21:02

The previous answers are indeed correct, but I think this is not about the for - it is about the comma, which is in fact an operator:

The comma operator evaluates both of its operands (from left to right) and returns the value of the second operand. - MDC

If you want to learn more about it: Angus C. elaborately explains what it does in his blog.

difference of use comma or semicomma

3 Answers3