function $(s) { if (document.getElementById) { return eval('document.getElementById("' + s + '")'); } else { return eval('document.all.' + s); } } //eval,這個(gè)比較常用
var myTest = function() { return "eval test"; };
function evalTest() {
  //簡(jiǎn)單數(shù)據(jù)
  alert(eval("1+1")); //2
  alert(eval("'a'+1")); //a1
  alert(eval("1+'a'")); //1a
  alert(eval("parseInt('a'+1)")); //NaN
  alert(eval("parseInt(1+'a')")); //1
  alert(eval("true")); //true
  alert(eval("0==false")); //true
  alert(eval("1==undefined")); //false
  alert(eval("isNaN(undefined)")); //true
  //函數(shù)和對(duì)象
  alert(eval("this")); //[object]
  alert(eval("typeof(this)")); //object
  alert(eval("typeof(test)")); //undefined
  alert(eval("evalTest")); //這個(gè)顯示當(dāng)前函數(shù)的定義語(yǔ)句細(xì)節(jié),包含注釋
  //alert(eval("evalTest()")); //調(diào)用自己并執(zhí)行,這個(gè)會(huì)有大問(wèn)題啊!
  alert(eval("typeof(evalTest)")); //function
  //其他
  var tmpFunc = "{a:1}";
  alert(eval(tmpFunc)); //1
  alert(eval("(" + tmpFunc + ")")); //[object object]
  alert(eval("tmpFunc")); //{a:1}
  //alert(eval("tmpFunc()")); //腳本錯(cuò)誤
  alert(myTest());
  eval("alert(myTest())"); //和上面等價(jià)
  alert(eval(myTest));
  alert(eval("myTest")); //和上面等價(jià)
  //form里的一個(gè)input,id=txtUserName
  eval("$('txtUserName').value='jeff wong';"); //等價(jià)于 $('txtUserName').value = 'jeff wong';
  eval("alert( $('txtUserName').value);");
}
evalTest(); 

var str = "global";
function test() {
  alert(str); //undefined
  var str = "local";
}
test();
alert(str); //global

var str = "global";
function test() {
  alert(str); //??
  eval("var str='local';");//會(huì)像我們預(yù)期的那樣嗎?
  //var str = "local";
  alert(str); //這里又是什么結(jié)果?
}
test();
alert(str); //??

var nav = new Object();
//通用eval函數(shù)
nav.Eval = function(jsCode) {
  if (document.all) //IE下是execScript
   execScript(jsCode);
  else window.eval(jsCode); //FF下是window.eval
}

var nav = new Object();
//通用eval函數(shù)
nav.Eval = function(jsCode) {
  if (document.all) //IE下是execScript
   execScript(jsCode);
  else window.eval(jsCode); //FF下是window.eval
}
function test() {
  nav.Eval("var str = 'global';"); //這里聲明變量str,在外面的函數(shù)中可以調(diào)用變量
  nav.Eval("var tmpFunc = function(){alert('global function');};"); //這里聲明函數(shù)變量tmpFunc,在外面的函數(shù)中可以調(diào)用函數(shù)
  alert(str); //global
  tmpFunc(); //global function
}
test();
alert(str); //global (調(diào)用nav.Eval函數(shù)聲明的全局變量)
tmpFunc(); // global function (調(diào)用nav.Eval函數(shù)聲明的全局函數(shù))

function withTest() {
 with (document) { //document的重復(fù)使用
 writeln("Hello,");
 writeln("it's a with keyword test!");
 }
 with (Math) { //Math的重復(fù)使用
 alert(random());
 alert(abs(-10));
 }
}
withTest();

function withTest() {
 var userName = "jeff wong";
 //暫時(shí)修改作用域鏈
 with (document) {
 writeln("Hello,");
 writeln(userName);
 }//with內(nèi)的語(yǔ)句執(zhí)行完之后，作用域鏈恢復(fù)原狀
 alert(userName);
}
withTest();