You would have to either:
A. Use a promise object
function getData(){
    return new Promise((resolve, reject) => {
         $.ajax({
            url:"<?= site_url('laporan/data_penjualan/') ?>",
            type:'POST',
            dataType:'JSON',
            success:(data) => resolve(data)
        });
     };
}
getData().then(data => {
    //data is your ajax response
});
B. Use a callback function if your website supports browsers that don't support Promise and you don't have a polyfill
$.ajax({
    url:"<?= site_url('laporan/data_penjualan/') ?>",
    type:'POST',
    dataType:'JSON',
    success: function(data){
        ajaxComplete(data);
    }
});
function ajaxComplete(data){
    //data is your ajax response
}