js判断方法变量是否存在

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<!DOCTYPE html><html lang="en"><head>    <meta charset="UTF-8">    <title>Title</title></head><body>//http://www.jb51.net/article/67551.htm//判断变量i是否存在 typeof(i)=="undefined"<script>    /*---------------------------判断函数是否存在-------------------------------*/    function isExitsFunction(funcName) {        try {            if (typeof(eval(funcName)) == "function") {                return true;                //   funcName();            }        } catch (e) {            console.log(eval(funcName) + "+++++++++++++++++我异常了!!!!!!!!");        }        return false;    }    /*--------------------------------判断是否存在指定变量 -----------------------------------------*/    function isExitsParamsVariable(variableName) {        try {            console.log("variableName.length===" + variableName.length);            if (variableName.length == 0) {                console.log(variableName + "===value has no params");//""：length为0                return false;            } else {                console.log(variableName + "======value has params");//0:length为undefined                return true;            }        } catch (e) {            console.log(variableName + "----我异常了!!!!!!!!");//null,undefined,未赋值的a        }        return false;//null,undefined,未赋值的a    }    /*---------------------------------判断是否undefined--------------------------------*/    function isExitsVariable(variableName) {        console.log("typeof variableName====" + typeof(variableName));        try {            if (typeof(variableName) == "undefined") {                console.log(variableName + "===value is undefined");//undefined,未赋值的a                return false;            } else {                console.log(variableName + "=======value is true");//null,0,""                return true;            }        } catch (e) {            console.log(variableName + "-------我异常了........");        }        return false;    }    /*-------------------------------------------------测试数据---------------------------------------------*/    var a;//声明未初始化，没有长度    console.log("isExitsParamsVariable(a)" + isExitsParamsVariable(a));    console.log("isExitsVariable(a)" + isExitsVariable(a));    console.log("--------------------------------------------------")    var b = undefined;//没有长度    console.log("isExitsParamsVariable(b)===" + isExitsParamsVariable(b));    console.log("isExitsVariable(b)===" + isExitsVariable(b));    console.log("--------------------------------------------------")    var c = null;//没有长度    console.log("isExitsParamsVariable(c)===" + isExitsParamsVariable(c));    console.log("isExitsVariable(c)===" + isExitsVariable(c));    console.log("--------------------------------------------------")    var d = 0;//长度undefined    console.log("isExitsParamsVariable(d)===" + isExitsParamsVariable(d));    console.log("isExitsVariable(d)===" + isExitsVariable(d));    console.log("--------------------------------------------------")    var e = "";//长度为0    console.log("isExitsParamsVariable(e)====" + isExitsParamsVariable(e));    console.log("isExitsVariable(e)===" + isExitsVariable(e));    console.log("--------------------------------------------------")    /*未定义声明 f 则log会报错：Uncaught ReferenceError: f is not defined ，不会执行两个判断方法*/    console.log("isExitsParamsVariable(f)====" + isExitsParamsVariable(f));//f:undefined    console.log("isExitsVariable(f)===" + isExitsVariable(f));</script></body></html>
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