为什么返回document.getElementById不起作用

Question

var $ = function (id){return document.getElementById(id);}
 window.onload=function(){
 $("num").onfocus=function(){
        $("show").style.display="block";
 }

Uncaught TypeError: undefined is not a function

but when i change $("num") to $("#num") it can works ,why?

Answer 1

You must have jQuery loaded on your page, and you define $ as a local variable, so during the onload the $ variable take it's old signification which is jQuery. And jQuery uses the same selectors as CSS.

You can solve your problem by disabling jQuery if you don't need it, or setting it in compatibility mode, or setting your $ variable as global variable.

Answer 2

Your code works if you

a) do not have jQuery loaded

b) add the missing curly bracket

Live demo

var $ = function (id){return document.getElementById(id);} 

window.onload=function(){
   $("num").onfocus=function(){
        $("show").style.display="block";
   }       
 }

Answer 3

You're most probably referencing uninformative jQuery $ variable name. And the reason is that both window.jQuery and window.$ are pointing to jQuery instance in case of referencing jQuery framework (sources) .

---

In fact, next should work as expected:

  window.onload=function(){ var $ = function (id){return document.getElementById(id);} $("num").onfocus=function(){ $("show").style.display="block"; } }