var myData = <?php echo json_encode($data, JSON_PRETTY_PRINT) ?>;

function setFilters(oldData) {

    var i = 0;
    var newItems = oldData;

    if (document.getElementById("isFreeFilter").checked){
        for (i = (newItems.length - 1); i >= 0; i--) {
            if (newItems[i].isFree != '1') {
                newItems.splice(i, 1);
            }
        }
    }
}

<input id="isFreeFilter" type="checkbox" name='chb1' value="1" onChange="setFilters(myData)">

newItems.splice(i, 1);

var newItems = oldData.slice();

var newItems = oldData.slice();

var newItems = oldData;