JavaScript判断变量与函数是否存在的正确方法

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<head>
 <meta charset="UTF-8">
 <title>Title</title>
</head>
<body>
////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>

判断变量是否已定义

if(!result){ 
 alert("发生错误"); 
} 

（1） 
 
if("undefined" == typeof result){ 
 alert("发生错误"); 
} 
（2） 
 
var result; 
if(undefined == result){ 
 alert("发生错误"); 
} 
（3） 
 
if("undefined" == typeof result){ 
 alert("发生错误"); 
} 

判断函数是否存在

<script>
//最常用
if("undefined" == typeof('a')){
//未定义
}else{
//定义
}

if("undefined" == typeof a){
//未定义
}else{
//定义
}

if(typeof a != "undefined"){
//true 定义
}else{
 //false 未定义
}
</script>

实际应用示例

if("undefined" != typeof downlm){ 
if(downlm=="soft"){ 
document.write('成功'); 
} 
}