What does a comma do in JavaScript expressions?

javascript comma-operator

Topera · Aug 24, 2010 · Viewed 11.5k times · Source

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.

What does the comma do in the last piece of code?

More Examples

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

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

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

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

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

console.log(1,2,3,4,5);
console.log((1,2,3,4,5));