ajax 同步請求和異步請求的差異分析

ajax同步和異步的差異， 先看2段代碼：
代碼一：

 代碼如下:

Synchronize = function(url,param) {
function createXhrObject() {
var http;
var activeX = [ "MSXML2.XMLHTTP.3.0", "MSXML2.XMLHTTP", "Microsoft.XMLHTTP" ];
try {
http = new XMLHttpRequest;
} catch (e) {
for (var i = 0; i < activeX.length; ++i) {
try {
http = new ActiveXObject(activeX[i]);
break;
} catch (e) {}
}
} finally {
return http;
}
}
var conn = createXhrObject();
conn.open("POST", url, false);//ajax同步
conn.send(param);
var strReturn = conn.responseText;
alert("1");
if (strReturn != "") {
return Ext.decode(conn.responseText);
} else {
return null;
}
alert("2");
};

代碼二：
Ajax 同步請求方式:

 代碼如下:

Synchronize = function(url,param) {
function createXhrObject() {
var http;
var activeX = [ "MSXML2.XMLHTTP.3.0", "MSXML2.XMLHTTP", "Microsoft.XMLHTTP" ];
try {
http = new XMLHttpRequest;
} catch (e) {
for (var i = 0; i < activeX.length; ++i) {
try {
http = new ActiveXObject(activeX[i]);
break;
} catch (e) {}
}
} finally {
return http;
}
}
var conn = createXhrObject();
conn.open("POST", url, true);//ajax異步
conn.send(param);
var strReturn = conn.responseText;
alert("1");
if (strReturn != "") {
return Ext.decode(conn.responseText);
} else {
return null;
}
alert("2");
};

同步和異步的差異如下：

conn.open('POST',Url,true); // ajax異步
conn.open('POST',Url,false); // ajax同步

對於代碼二，爲異步的ajax請求，執行結果爲：先執行alert(2)再執行alert(1), 異步的意思就是說一旦conn.open請求一發出，前端不去等待它的響應便執行後面的代碼，所以alert(2)先執行了，然後當響應response到達以後才執行alert(1);

對於代碼一，爲同步的ajax請求，執行結果爲：先執行alert(1)再執行alert(2), 同步的意思就是說一旦conn.open請求一發出，前端就去等待它的響應，響應完成以後，alert(1)先執行了，然後alert(2);