1) Use three ways to locate elements in html
a) by ID
$("#ID")
b) by tag name
$("tag name")
c) by style name
$(".style name")
2) dom In , it is necessary to judge whether the element found is null,
but in jquery, there is no need to judge, because jquery itself has a built-in judging device, if it can't find it, it returns "undefined"
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN">
<html>
<head>
<meta http-equiv="content-type" content="text/html; charset=UTF-8">
<script type="text/javascript" src="../js/jquery-1.6.js">
</script>
<style type="text/css">
.oneClass{
font-size:44px;
color:red
}
</style>
<body>
<input type="text" id="usernameID" value="这就是jQuery"/>
<div id="divID" class="oneClass">这是div中的内容</div>
<script type="text/javascript">
//通过"#id"定位<input>
//alert($("#usernameID").val());
//通过"标签名"定位<input><div>
//alert($("div").html());
//alert($("input").val());
//通过"样式名"定位<div>
//alert($(".oneClass").html());
//dom错误处理和jquery错误处理
var $input = $("#usernameIDD");
alert($input.val());
/*
var inputElement = document.getElementById("usernameID");
if(inputElement!=null){
alert(inputElement.value);
}else{
alert("查无此元素");
}
*/
</script>
</body>
</html>