Question

I read an e-book and find this command, but I don’t know it’s purpose and when to using it. Example:

var x = (12, 34, 56)

x in this case is a number and it’s value is 56. Thanks.

This is the full script I read from ebook ‘Javascript for absolute Beginners’:

var fridge = {
buttermilk: [1/3, "cup"],
kefir: [1 + 1/2, "cup"],
yogurt: [4, "cup"],
};
var dough = {
hardWhiteWholeWheatFlour: [2, "cup"],
sugar: [1/3, "cup"],
madagascarVanilla: [1, "tsp"],
orangeZest: [1, "tbs"],
soda: [1, "tsp"],
tartar: [1, "tsp"],
orangeJuice: [1/2, "cup"],
culturedMilk: [1/2, "cup"],
egg: [1],
cranberries: [2/3, "cup"]
};
dough.culturedMilk[0] = fridge.buttermilk[0] >= 1/2 ? (fridge.buttermilk[0] -= 1/2, 1/2) :
fridge.kefir[0] >= 9/16 ? (fridge.kefir[0] -= 9/16, 9/16) :
fridge.yogurt[0] >= 10/16 ? (fridge.yogurt[0] -= 10/16, 10/16) :
alert("No cranberry bread for you!");
dough.culturedMilk;
// [0.5625, "cup"]
fridge.kefir;
// [0.9375, "cup"]

Answer 1

It seems you already know what it means… evaluated each operand and return the result of the last.

In the case of (fridge.buttermilk[0] -= 1/2, 1/2), the expression itself is part of the conditional operator:

fridge.buttermilk[0] >= 1/2 ? (fridge.buttermilk[0] -= 1/2, 1/2) : ...

In this case, the comma operator is used to introduce side effects. If fridge.buttermilk[0] >= 1/2 is true, then 1/2 should be assigned to dough.culturedMilk[0]. But at the same time, fridge.buttermilk[0] should be updated as well.

Normally you would use two expression statements, like

if(fridge.buttermilk[0] >= 1/2) {
    fridge.buttermilk[0] -= 1/2;
    dough.culturedMilk[0] = 1/2;
}

but since the author wants to use the conditional operator, the comma operator can be used to execute both expression in one expression. It updates the value of fridge.buttermilk[0] and returns 1/2.

I’m not recommending doing this though, especially in this case, with nested conditional operators, a traditional if-else statement would be easier to read.