JavaScript中如何判断函数、变量是否存在

try

{

 if(typeof(eval(funcName))=="function")

 {

   funcName();

 }

}catch(e)

{

//alert("not function");

}

function isExitsVariable(variableName) {

  try {

    if (typeof(variableName) == "undefined") {

      //alert("value is undefined");

      return false;

    } else {

      //alert("value is true");

      return true;

    }

  } catch(e) {}

  return false;

}

if("undefined" != typeof downlm){

if(downlm=="soft"){

document.write('成功');

}

}