[jquery] Datatables Select All Checkbox

This should work for you:

let example = $('#example').DataTable({
    columnDefs: [{
        orderable: false,
        className: 'select-checkbox',
        targets: 0
    }],
    select: {
        style: 'os',
        selector: 'td:first-child'
    },
    order: [
        [1, 'asc']
    ]
});
example.on("click", "th.select-checkbox", function() {
    if ($("th.select-checkbox").hasClass("selected")) {
        example.rows().deselect();
        $("th.select-checkbox").removeClass("selected");
    } else {
        example.rows().select();
        $("th.select-checkbox").addClass("selected");
    }
}).on("select deselect", function() {
    ("Some selection or deselection going on")
    if (example.rows({
            selected: true
        }).count() !== example.rows().count()) {
        $("th.select-checkbox").removeClass("selected");
    } else {
        $("th.select-checkbox").addClass("selected");
    }
});

I've added to the CSS though:

table.dataTable tr th.select-checkbox.selected::after {
    content: "?";
    margin-top: -11px;
    margin-left: -4px;
    text-align: center;
    text-shadow: rgb(176, 190, 217) 1px 1px, rgb(176, 190, 217) -1px -1px, rgb(176, 190, 217) 1px -1px, rgb(176, 190, 217) -1px 1px;
}

Working JSFiddle, hope that helps.