SOURCE

function getBank_account() {
    var bank_no = document.getElementById("bank_no_select").value;
    var prefix = "";
    switch (bank_no) {
        case "0102":
            prefix = "622202";
            break;
        case "0103":
            prefix = "622848";
            break;
        case "0105":
            prefix = "622700";
            break;
        case "0301":
            prefix = "622262";
            break;
        case "104":
            prefix = "621661";
            break;
        case "0303":
            prefix = "622666";
            break;
        case "305":
            prefix = "622622";
            break;
        case "0306":
            prefix = "622556";
            break;
        case "0308":
            prefix = "622588";
            break;
        case "0410":
            prefix = "622155";
            break;
        case "302":
            prefix = "622689";
            break;
        case "304":
            prefix = "622630";
            break;
        case "309":
            prefix = "622908";
            break;
        case "310":
            prefix = "621717";
            break;
        case "315":
            prefix = "622323";
            break;
        case "316":
            prefix = "622309";
            break;
        default:
    }
    for (var j = 0; j < 13; j++) {
        prefix = prefix + Math.floor(Math.random() * 10);
    }
    var x = document.getElementsByName("bank_no");
    for (var i = 0; i < x.length; i++) {
        var o = x[i];
        o.value = bank_no;
    }
    var y = document.getElementsByName("bank_account");
    for (var i = 0; i < y.length; i++) {
        var o = y[i];
        o.value = prefix;
    }
}