Javascript syntax: what comma means?

Question

If I use:

1.09 * 1; // returns "1.09"

But if I use:

1,09 * 1; // returns "9"

I know that 1,09 isn't a number. So, what comma does in the last piece of code?

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EDIT 1

Another example:

if(0,9) alert("ok"); // alert

But

if(9,0) alert("ok"); // don't alert

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EDIT 2

Yet Another example:

alert(1);alert(2);alert(3); // 3 alerts

And

alert(1),alert(2),alert(3); // 3 alerts too

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EDIT 3

Crazy example:

alert("2",
    foo = function (param){
        alert(param)
    },
    foo('1')
)
foo('3'); // alerts 1, 2 and 3

Answer 1

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

Source: https://developer.mozilla.org/en/JavaScript/Reference/Operators/Special_Operators/Comma_Operator

For example, the expression 1,2,3,4,5 evaluates to 5. Obviously the comma operator is useful only for operations with side-effects.

Answer 2

Have a look here - the comma stands for multiple expressions / statements. For example in your code you could use a line like this:

var a=0, b=0, c=0;

This would declare all three variables without writing:

var a=0;
var b=0;
var c=0;

Hope that helps.

Regards, Daniel

Answer 3

Some more alerts to consider:

alert((0, 9));
alert((9, 0));
alert(("foo", "bar"));

Also, have a look at the Firebug console if you want to try this out interactively.