What is the purpose for multiple comma-separated expressions in a WHILE condition?

Question

I stumbled over the following JavaScript:

Code:

var x="", i=0;
while (i<4, i<7, i<5, i<6)
{
  x=x + "The number is " + i + "<br>";
  i++;
}
document.write(x);

I never know that it is possible to use multiple comma-separated expression in one WHILE statement.

Result:

The number is 0
The number is 1
The number is 2
The number is 3
The number is 4
The number is 5

What is the reason for this and what is the behavior (only the last expression is evaluated for the while-exit)?

Is the behavior standardize in all Javascript/C/Java languages?

Answer 1

This is a special operator in JavaScript which is called comma operator; so when JavaScript comes across this line

(i<4, i<7, i<5, i<6)

it will evaluate one by one all the operands in the parenthesis and will return the evaluation of the last item i.e. i < 6

It is rarely used; One occasion that I remember to have used it was for indirect call to eval.

(0, eval)(src)

But again in you example the use of it is redundant.

Answer 2

Only the last result is kept in result of the comma operator, so

while (i<4, i<7, i<5, i<6)

is really equivalent to

while (i<6)

That's the reason why you don't see this more often : it's useless if your comparisons don't have side effects.