When is the comma operator useful?

The following is probably not very useful as you don't write it yourself, but a minifier can shrink code using the comma operator. For example:

if(x){foo();return bar()}else{return 1}

would become:

return x?(foo(),bar()):1

The ? : operator can be used now, since the comma operator (to a certain extent) allows for two statements to be written as one statement.

This is useful in that it allows for some neat compression (39 -> 24 bytes here).

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I'd like to stress the fact that the comma in var a, b is not the comma operator because it doesn't exist within an expression. The comma has a special meaning in var statements. a, b in an expression would be referring to the two variables and evaluate to b, which is not the case for var a, b.

The comma operator allows you to put multiple expressions in a place where one expression is expected. The resulting value of multiple expressions separate by a comma will be the value of the last comma separated expression.

I don't personally use it very often because there aren't that many situations where more than one expression is expected and there isn't a less confusing way to write the code than using the comma operator. One interesting possibility is at the end of a for loop when you want more than one variable to be incremented:

// j is initialized to some other value
// as the for loop executes both i and j are incremented
// because the comma operator allows two statements to be put in place of one
for (var i = 0; i < items.len; i++, j++) {
    // loop code here that operates on items[i] 
    // and sometimes uses j to access a different array
}

Here you see that i++, j++ can be put in a place where one expression is allowed. In this particular case, the multiple expressions are used for side affects so it does not matter that the compound expressions takes on the value of the last one, but there are other cases where that might actually matter.