jQuery中的Hide（）不起作用

Code in index.html

<head>
<script type="text/javascript" src="js/jquery.min.js"></script>
<script type="text/javascript" src="js/index.js"></script>
</head>

<body onload="loadIndex()">
<a id="registration" href="register.html"> Register </a>
</body>

Code in index.js

function loadIndex() {
alert("Page is loaded");
$("#registration").hide();
}

I get alert, but link is not hidden. I have more files, and this does not work in any of them. I think that mistake is not in code, but I don't know where is.

Your code is fine with the exception of a tiny syntax issue. You're just missing the closing tag </head>. Check code below:

function loadIndex() {
  alert("Page is loaded");
  $("#registration").hide();
}
<script src="https://ajax.googleapis.com/ajax/libs/jquery/2.1.1/jquery.min.js"></script>
<head>
  <script type="text/javascript" src="js/jquery.min.js"></script>
  <script type="text/javascript" src="js/index.js"></script>
</head>

<body onload="loadIndex()">
  <a id="registration" href="register.html"> Register </a>
</body>

</div>

Try this setup:

<head>
<script type="text/javascript" src="js/jquery.min.js"></script>
<script type="text/javascript" src="js/index.js"></script>
  <script>
     $(document).ready(function(){
       alert("Page is loaded");
       $("#registration").hide();
     });
  </script>
</head>
<body>
   <a id="registration" href="register.html"> Register </a>
</body>

I had same problem with jQuery v1.11 before, I solved this problem by creating new class in my css file as below :

.hidden {
  display : none;
}

and for show and hide elem, I added and removed hidden class from elem.

hide: $(elem).addClass('hidden');

show: $(elem).removeClass('hidden');