怎样使用JS判断变量是否存在

来源：尚港娱乐

这次给大家带来怎样使用JS判断变量是否存在，使用JS判断变量是否存在的注意事项有哪些，下面就是实战案例，一起来看一下。

<!DOCTYPE html>
<html lang="en">
<head>
 <meta charset="UTF-8">
 <title>Title</title>
</head>
<body>
//http://www.gxlcms.com/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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怎样使用JS判断变量是否存在