How to pass a JS array to PHP array

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Javascript

PHP

I would like to convert a JS array to a PHP array. I tried to send the value to the PHP page:

var seatsReserved = [];

function reserve(seat){
seat.setAttribute("class","reserved");
var hasClass = seat.classList.contains('reserved');
if (hasClass) {
seatsReserved.push(parseInt(seat.innerHTML));

$.post('../index.php', { 'seatsReserved': JSON.stringify(seatsReserved) });
console.log(seatsReserved.sort(function(a, b){return a-b}));
}
}

However, when I run

$seatsReserved = json_decode($_POST['seatsReserved']);

I get this error:

Notice: Undefined index: seatsReserved

How can I correctly pass the array to PHP?