JavaScript中如何判断函数、变量是否存在

function isExitsFunction(funcName) {
  try {
    if (typeof(eval(funcName)) == "function") {
      return true;
    }
  } catch(e) {}
  return false;
}

if (typeof String.prototype.endsWith != 'function') {
 String.prototype.endsWith = function(suffix) {
  return this.indexOf(suffix, this.length - suffix.length) !== -1;
 };
}

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'); 
} 
}