The comma operator evaluates each of its operands (from left to right) and returns the value of the last operand.
expr1, expr2, expr3...
expr1, expr2, expr3...You can use the comma operator when you want to include multiple expressions in a location that requires a single expression. The most common usage of this operator is to supply multiple parameters in a for loop.
If a is a 2-dimensional array with 10 elements on a side, the following code uses the comma operator to increment two variables at once. Note that the comma in the var statement is not the comma operator, because it doesn't exist within an expression. Rather, it is a special character in var statements to combine multiple of them into one. Practically, that comma behaves almost the same as the comma operator, though. The code prints the values of the diagonal elements in the array:
for (var i = 0, j = 9; i <= 9; i++, j--)
document.writeln("a[" + i + "][" + j + "] = " + a[i][j]);
Another example that one could make with comma operator is processing before returning. As stated, only the last element will be returned but all others are going to be evaluated as well. So, one could do:
function myFunc () {
var x = 0;
return (x += 1, x); // the same as return ++x;
}
Created by Mozilla Contributors, license: CC-BY-SA 2.5