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How to use JS to determine whether a variable exists

php中世界最好的语言Original: 2018-05-29 10:00:492771browse

This time I will show you how to use JS to determine whether a variable exists, and what are the precautions for using JS to determine whether a variable exists. The following is a practical case, let's take a look.

<!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>

I believe you have mastered the method after reading the case in this article. For more exciting information, please pay attention to other related articles on the php Chinese website!

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