functiontestabc(a,b,c){a=typeofa!=='undefined'?a:'1';b=typeofb!=='undefined'?b:'2';c=typeofc!=='undefined'?c:'3';alert(b);}testabc(b='b');JS不能這么用嗎？類似$.ajax這種用法是如何實(shí)現(xiàn)的呢？$.ajax({url:'xxx'});