<div class="bg">
 <div id="txt"></div>
 <button id="bt10" value="+">+</button>
 <button id="bt11" value="-">-</button>
 <button id="bt12" value="*">*</button>
 <br data-filtered="filtered"><button id="bt13" value="/">/</button>
 <button id="bt14" value="=">=</button>
 <br data-filtered="filtered"><button id="bt1" value="1">1</button>
 <button id="bt2" value="2">2</button>
 <button id="bt3" value="3">3</button>
 <br data-filtered="filtered"><button id="bt4" value="4">4</button>
 <button id="bt5" value="5">5</button>
 <button id="bt6" value="6">6</button>
 <br data-filtered="filtered"><button id="bt7" value="7">7</button>
 <button id="bt8" value="8">8</button>
 <button id="bt9" value="9">9</button> 
</div>

var content; //存儲(chǔ)已點(diǎn)擊的數(shù)字或符號(hào)，要定義成全局的，如果定義在number()中，每次content都會(huì)被重新賦值
function number(z) {
  var k=document.getElementById("txt");//獲取顯示框的 DOM，并緩存在k中
  if(z==14){//如果點(diǎn)擊了"="號(hào)
   var sum = fact(content);//調(diào)用fact()進(jìn)行計(jì)算，并把結(jié)果賦值給sum
   content=content+"="+sum;//在要顯示的內(nèi)容后加入"="和sum
   k.innerHTML = content;
   content = null;//將content清空，準(zhǔn)備下次計(jì)算
  }else{
  //如果沒(méi)有點(diǎn)擊"="號(hào)，而是點(diǎn)擊的運(yùn)算符，就需要通過(guò)switch把數(shù)字轉(zhuǎn)化成運(yùn)算符
   switch(z){
    case 10: z = '+'; break;
    case 11: z = '-'; break;
    case 12: z = '*'; break;
    case 13: z = '/'; break;
   }
   //把此時(shí)輸入的字符存入content
   if(content){
    content+=z.toString();
   }else{
    content=z.toString();
   }
   k.innerHTML = content;//讓它實(shí)時(shí)顯示
  }
}

//實(shí)現(xiàn)遞歸計(jì)算
function fact(content){
 var index = content.indexOf("+");//獲取"+"號(hào)的index
 if(index != -1){
  return fact(content.substring(0,index))+fact(content.substring(index+1));
  //當(dāng)找得到“＋”號(hào)時(shí)，分成兩部分相加遞歸
 }else{
  var index2 = content.lastIndexOf("-");//當(dāng)找不到“＋”號(hào)時(shí)，開始找“－”號(hào)
  if(index2 != -1){
   return fact(content.substring(0,index2))-fact(content.substring(index2+1));
   //當(dāng)找得到“－”號(hào)時(shí)，分成兩部分相減遞歸
  }else{
   var index3 = content.indexOf("*");//當(dāng)找不到“－”號(hào)時(shí)，開始找“＊”號(hào)
   if(index3 != -1){
    return fact(content.substring(0,index3))*fact(content.substring(index3+1));
    //當(dāng)找得到“＊”號(hào)時(shí)，分成兩部分相乘遞歸
   }else{
    var index4 = content.lastIndexOf("/");//當(dāng)找不到“＊”號(hào)時(shí)，開始找“/”號(hào)
    if(index4 != -1){
     return fact(content.substring(0,index4))/fact(content.substring(index4+1));
     //當(dāng)找得到“/”號(hào)時(shí)，分成兩部分相除遞歸
    }else{
     return parseFloat(content);//當(dāng)找不到“/”號(hào)時(shí)，返回這段串的變成float型的數(shù)值
    }
   }
  }
 }
}