【ajax】xhr

jQuery

xhr: function() {
    return window.ActiveXObject ? new ActiveXObject("Microsoft.XMLHTTP") : new XMLHttpRequest();
}

JavaScript框架设计

// 缓存
function xhr() {
    if (!xhr.cache) {
        var fns = [
            function () { return new XMLHttpRequest(); },
            function () { return new ActiveXObject('Msxml2.XMLHTTP'); },
            function () { return new ActiveXObject('Microsoft.XMLHTTP'); },
        ];
        for (var i = 0, n = fns.length; i < n; i++) {
            try {
                fns[i]();
                xhr.cache = fns[i];
                break;
            } catch(e) {}
        }
        return xhr.cache();
    } else {
        return xhr.cache();
    }
}

var xhrObject = xhr(); //调用
alert(xhrObject) //[object XMLHttpRequest]

var xhr = function() {
    var fns = [
        function () { return new XMLHttpRequest(); },
        function () { return new ActiveXObject('Msxml2.XMLHTTP'); },
        function () { return new ActiveXObject('Microsoft.XMLHTTP'); },
    ];
    for (var i = 0, n = fns.length; i < n; i++) {
        try {
            fns[i]();
            xhr = fns[i];//注意这里，覆写自身
            break;
        }catch(e) {}
    }
    return xhr()
}

console.log(xhr)
console.log(xhr())
console.log(xhr)
console.log(xhr())

window.$ = {}
var s = ["XMLHttpRequest", "ActiveXObject('Msxml2.XMLHTTP.6.0')","ActiveXObject('Msxml2.XMLHTTP.3.0')", "ActiveXObject('Msxml2.XMLHTTP')"];
if (!"1"[0]) {
    //判定IE67
    s[0] = location.protocol === "file:" ? "!" : s[0]; // 报异常，跳过
}
for (var i = 0, axo; axo = s[i++]; ) {
    try {
        if (eval("new " + axo)) {
            $.xhr = new Function("return new " + axo);
            break;
        }
    } catch (e) {}
}

我们再认真思考一下，既然我们是写框架，那么这些检测其实是放在 IIFE 里面，因此基本不用覆写，检测好哪个可用，就把它加到命名空间上就好了。最后的版本就出来了，使用new Function、eval，反正只用一次，耗不了多少性能