封装AJAX库（参考JQ）

　　//jQ方法 $.ajax([URL],[OPTIONS])

$.ajax({
 url:'',
 data:null,
 datatype:'json',
 method:'GET',
 async:true,
 cache:true,
 success:()=>{},
 error:()=>{}
})

 //封装成window对象的一个方法。

(function anonymous(window) {
    function AJAX(options) {
        return new AJAX.prototype.init(options)
    }
    let init = function init(options = {}){
        let {
            url,
            method = 'GET', //默认值用等号
            data = null,
            datatype = 'JSON',
            async = true,
            cache = true,
            success,
            error

        } = options;
        //MOUNT :将配置挂载到实例上
        ['url', 'method', 'data', 'datatype', 'async', 'cache', 'success',
            'error'
        ].forEach((item) => {
            this[item] = eval(item);
        });
        //SEND :发送请求
        this.sendAjax();
    };
    AJAX.prototype = {
        constructor: AJAX,
        init,
        //发送AJAX请求
        sendAjax() {
            this.handleDate();
            this.handleCache();
            let (method, url, async,error,success,data) = this;
            //四部曲
            let xhr = new XMLHttpRequest();
            xhr.open(method, url, async);
            xhr.onreadystatechange = () => {
                if (!/^(2|3)d{2}$/.test(xhr.status)) {
                    error && error(xhr.statusText, xhr);
                    return;
                }
                if (xhr.readyState === 4) {
                    let result = this.handleDataType(xhr);
                    success && success(result, xhr);
                }
            };
            xhr.send();
        },
        //处理DATA-TYPE
        handleDataType(xhr) {
            let dataType = this.dataType.toUpperCase();
            result = xhr.responseText;
            switch (dataType) {
                case 'TEXT':
                    break;
                case 'JSON':
                    result = JSON.parse(result);
                    break;
                case 'XML':
                    xhr.responseXML;
                    break;
            }
            return result;
        },
        //处理CACHE
        handleCache() {
            let {
                url,
                method,
                cache
            } = this;
            if (/^GET$/i.test(method) && cache === false) {
                url += `${check()}_=${+new Date()}`; //
            }
        },
        //处理DATA
        handleData(){
            let {data,method} = this;
            if(!data) return;
            let str = '';
            if(typeof data === 'object'){
                //如果是对象，转换成x-www-form-urlencoded模式的字符串
                for(let key in data){
                    if(data.hasOwnProperty(key)){
                        str += `${key}=${data[key]}&`;
                    }
                }
                data = str.substring(0,str.length-1);
            }
            //根据请求方式的不同，传递给服务器的也不同
            if(/^(GET|DELETE|HEAD|TRACT|OPTIONS)$/i.test(method)){
                this.url += `${this.check()}${data}`;
                this.data = null;
                return;
            }
            //当请求为POST时
            this.data = data;    
        },
        //检测问号
        check() {
            return this.url.indexOf('?')>-1?'&':'?';
        }
    }
    init.prototype = AJAX.prototype;
    window.ajax = AJAX;
})(window)
//直接ajax(options),options为一个对象