上次学ajax,感觉没什么味，然后就跳过了。今天重新把它拾起。

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<div>
	<div id="showInfo"></div>
	<form action="">
		用户名：<input type="text" name="username" id="username"><br>
		密  码：<input type="password" name="password" id="password">
		<input type="button" value="登录" id="btn">
	</form>
</div>
	<script>
		var btn = document.getElementById('btn');


		btn.onclick = function() {
			var username = document.getElementById('username').value;
			var password = document.getElementById('password').value;

			var xhr = null;
			
			//兼容代码
			if(window.XMLHttpRequest) {
				xhr = new XMLHttpRequest();
			} else {
				xhr = new ActiveXObject("Mircsoft.XML");
			}

			var url = './check.php?username='+username+"&password="+password;
			xhr.open('get', url, true);

			xhr.onreadystatechange = function() {
				if(xhr.readyState == 4 && xhr.status == 200) {
					var data = xhr.responseText;
						if (data == 1) {
							document.getElementById('showInfo').innerHTML = '用户名或者密码错误';
						} else if (data == 2) {
							document.getElementById('showInfo').innerHTML = '登录成功！';
						}

				}
			}
			xhr.send(null);
		}
	</script>