{})
//如果它没有定义$.ajaxSettings里面的属性的时候，才去将$.ajaxSettings[key] 复制过来
for (key in $.ajaxSettings) if (settings[key] === undefined) settings[key] = $.ajaxSettings[key]
//执行全局ajaxStart
ajaxStart(settings)
//通过判断请求地址和当前页面地址的host是否相同来设置是跨域
if (!settings.crossDomain) settings.crossDomain = /^([\w-]+:)?\/\/([^\/]+)/.test(settings.url) && RegExp.$2 != window.location.host
//如果没有设置请求地址，则取当前页面地址
if (!settings.url) settings.url = window.location.toString();
//将data进行转换
serializeData(settings);
//如果不设置缓存
if (settings.cache === false) settings.url = appendQuery(settings.url, '_=' + Date.now())
//如果请求的是jsonp，则将地址栏里的=?替换为callback=?,相当于一个简写
var dataType = settings.dataType,
hasPlaceholder = /=\?/.test(settings.url)
if (dataType == 'jsonp'
关键词：zepto是啥