从jquery，zepto中提取的90行代码实现一个完整的ajax

function ajax(options) {
    function empty() {}

    function obj2Url(obj) {
        if (obj && obj instanceof Object) {
            var arr = [];
            for (var i in obj) {
                if (obj.hasOwnProperty(i)) {
                    if (typeof obj[i] == 'function') obj[i] = obj[i]();
                    if (obj[i] == null) obj[i] = '';
                    arr.push(escape(i) + '=' + escape(obj[i]));
                }
            }
            return arr.join('&').replace(/%20/g, '+');
        } else {
            return obj;
        }
    };
    var opt = {
        url: '', //请求地址
        sync: true, //true，异步 | false　同步，会锁死浏览器，并且open方法会报浏览器警告
        method: 'GET', //提交方法
        data: null, //提交数据
        username: null, //账号
        password: null, //密码
        dataType: null, //返回数据类型
        success: empty, //成功返回回调
        error: empty, //错误信息回调
        timeout: 0, //请求超时ms
    };
    for (var i in options) if (options.hasOwnProperty(i)) opt[i] = options[i];
    var accepts = {
        script: 'text/javascript, application/javascript, application/x-javascript',
        json: 'application/json',
        xml: 'application/xml, text/xml',
        html: 'text/html',
        text: 'text/plain'
    };
    var abortTimeout = null;
    var xhr = new XMLHttpRequest();
    xhr.onreadystatechange = function() {
        if (xhr.readyState == 4) {
            xhr.onreadystatechange = empty;
            clearTimeout(abortTimeout);
            var result,dataType, error = false;
            if ((xhr.status >= 200 && xhr.status < 300) || xhr.status == 304 || (xhr.status == 0 && protocol == 'file:')) {
                if (xhr.responseType == 'arraybuffer' || xhr.responseType == 'blob') {
                    result = xhr.response;
                } else {
                    result = xhr.responseText;
                    dataType = opt.dataType ? opt.dataType : xhr.getResponseHeader('content-type').split(';', 1)[0];
                    for (var i in accepts) {
                        if (accepts.hasOwnProperty(i) && accepts[i].indexOf(dataType) > -1) dataType = i;
                    }
                    try {
                        if (dataType == 'script') {
                            eval(result);
                        } else if (dataType == 'xml') {
                            result = xhr.responseXML
                        } else if (dataType == 'json') {
                            result = result.trim() == '' ? null : JSON.parse(result)
                        }
                    } catch (e) {
                        opt.error(e, xhr);
                        xhr.abort();
                    }
                }
                opt.success(result, xhr);
            } else {
                opt.error(xhr.statusText, xhr);
            }
        }
    };
    if (opt.method == 'GET') {
        var parse = opt.url.parseURL();
        opt.data = Object.assign({}, opt.data, parse.params);
        opt.url = parse.pathname + '?' + obj2Url(opt.data);
        opt.data = null;
    }
    xhr.open(opt.method, opt.url, opt.sync, opt.username, opt.password);
    if (opt.method == 'POST') xhr.setRequestHeader('Content-type', 'application/x-www-form-urlencoded');
    if (opt.timeout > 0) {
        abortTimeout = setTimeout(function() {
            xhr.onreadystatechange = empty
            xhr.abort();
            opt.error('timeout', xhr);
        }, opt.timeout)
    }
    xhr.send(opt.data ? obj2Url(opt.data) : null);
}

test

ajax({
    url:'/url',
    data:{
        i:new Date().getTime(),
    },
    success:function(data,xhr){
        console.log(data,xhr);
    },
    error:function(err,xhr){
        console.log(err,xhr);
    }
});