function createRequest() {	
	try {
		request = new XMLHttpRequest();
	}catch (tryMS) {
		try {
			request = new ActiveXObject("Msxml2.XMLHTTP");
		}catch (otherMS) {
			try {
				request = new ActiveXObject("Microsoft.XMLHTTP");
			}catch (Failed) {
				request = null;
			}
		}
	}
	
	return request;
}

此函数在创建异步请求时考虑了各种浏览器的差异，是一个通用函数。