javascript小陷阱

//有点意思

case 1:
functiona(x) {

console.log(x);

function b() {

var x= x || 8;

console.log(x);

}

b()

} 

 a(3);//猜猜？

 

case 2: 

function a(x) {

console.log(x);

function b() {

var x;

x= x || 8;

console.log(x);

}

b()

}

 

a(3); //猜猜？

 

 

结果是一样的，都是3,8；为什么不是3,3呢？原因也很简单，函数b中的var x在作怪，使得在b中x的值首先变成了undefined,

var x=x||8;与var x;x=x||8 是等价的，前面那种形式实际上会先被js引擎转换成后者， 

自然x=undefined||8会得到x=8。

 

之所以提到这个小问题，是因为在一个项目中不小心写了这样的代码： 

var cache={
readServer:function (callback) {
            var paramsObj = {
                lemmaId:commonSubmitData.lemmaId,
                versionId:commonSubmitData.versionId,
                subLemmaId:dynamicData.activeSubLemmaId,
                subVersionId:dynamicData.activeSubVersionId,
                type:1
            };
            var paramsStr = baidu.url.jsonToQuery(paramsObj),
                self = this;

            initData.communicating = true;
            ajax.post(initData.doubtShowUrl, paramsStr, function (xhr, msg) {
                initData.firstClick = false;
                initData.communicating = false;
                if (!self.doubtDataCache[dynamicData.activeSubLemmaId]) {
                    self.doubtDataCache[dynamicData.activeSubLemmaId] = json.parse(msg);
                }

//下面两行是错误的
var callback=callback||function(){};
callback.call(self, self.doubtDataCache[dynamicData.activeSubLemmaId]);
/***这里是正确的
                if(lang.isFunction(callback)){
                    callback.call(self, self.doubtDataCache[dynamicData.activeSubLemmaId]);
                }
*/
                //console.log(callback);
            });
        }

}