想问一下JQuery ajax如何解决浏览器的兼容性问题

Question

图片是用js实现ajax的一个样例，可以看到用了很多判断去实现浏览器的兼容以及服务器
状态等，但是今天看JQuery实现ajax时，没有这些判断。我在网上看到JQuery实现的ajax
已经能够兼容浏览器了，所以我想问下这部分的内容在哪里优化的？

Answer 1

你的这些代码jquery已经写到js文件里面了，自己看未压缩的jquery文件不就清楚了
https://code.jquery.com/jquery-1.12.4.js

 // Functions to create xhrs
function createStandardXHR() {
    try {
        return new window.XMLHttpRequest();
    } catch ( e ) {}
}

function createActiveXHR() {
    try {
        return new window.ActiveXObject( "Microsoft.XMLHTTP" );
    } catch ( e ) {}
}

jQuery.ajaxSettings.xhr = window.ActiveXObject !== undefined ?

    // Support: IE6-IE8
    function() {

        // XHR cannot access local files, always use ActiveX for that case
        if ( this.isLocal ) {
            return createActiveXHR();
        }

        // Support: IE 9-11
        // IE seems to error on cross-domain PATCH requests when ActiveX XHR
        // is used. In IE 9+ always use the native XHR.
        // Note: this condition won't catch Edge as it doesn't define
        // document.documentMode but it also doesn't support ActiveX so it won't
        // reach this code.
        if ( document.documentMode > 8 ) {
            return createStandardXHR();
        }

        // Support: IE<9
        // oldIE XHR does not support non-RFC2616 methods (#13240)
        // See http://msdn.microsoft.com/en-us/library/ie/ms536648(v=vs.85).aspx
        // and http://www.w3.org/Protocols/rfc2616/rfc2616-sec9.html#sec9
        // Although this check for six methods instead of eight
        // since IE also does not support "trace" and "connect"
        return /^(get|post|head|put|delete|options)$/i.test( this.type ) &&
            createStandardXHR() || createActiveXHR();
    } :

    // For all other browsers, use the standard XMLHttpRequest object
    createStandardXHR;