$(document).ready(function () {
    function_01();
    $("#btn_submit").click(function_02);

});

//page load時 會自動觸發
var function_01 = function () {
   //do something
};

//$("#id").click時觸發
var function_02 = function () {
    //do something
};

延伸閱讀：常用的jQuery事件觸發使用↓↓↓↓↓↓↓
http://www.w3school.com.cn/jquery/jquery_ref_events.asp

2018-12-26 $(document).ready(function () { ... }); == $(function () { ... });

$(document).ready(function () {
    var $targetObj = $('#targetObj');
    //初始化設定
    cat.touchjs.init($targetObj, function (left, top, scale, rotate) {
        //...
    });
    //初始化拖動手勢
    cat.touchjs.drag($targetObj, function (left, top) {
        //...
    });
    //初始化縮放手勢
    cat.touchjs.scale($targetObj, function (scale) {
        //...
    });
    //初始化旋轉手勢
    cat.touchjs.rotate($targetObj, function (rotate) {
        //...
    });
});

//--------------------------------------

$(function () {
    var $targetObj = $('#targetObj');
    //初始化设置
    cat.touchjs.init($targetObj, function (left, top, scale, rotate) {
        //...
    });
    //初始化拖動手勢
    cat.touchjs.drag($targetObj, function (left, top) {
        //...
    });
    //初始化縮放手勢
    cat.touchjs.scale($targetObj, function (scale) {
        //...
    });
    //初始化旋轉手勢
    cat.touchjs.rotate($targetObj, function (rotate) {
        //...
    });
});