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JavaScript數(shù)據(jù)類型轉(zhuǎn)換的注意事項(xiàng)

 更新時(shí)間:2016年07月31日 11:53:19   作者:埃索達(dá)短劍  
JavaScript是一種無(wú)類型語(yǔ)言,但同時(shí)JavaScript提供了一種靈活的自動(dòng)類型轉(zhuǎn)換的處理方式?;疽?guī)則是,如果某個(gè)類型的值用于需要其他類型的值的環(huán)境中,JavaScript就自動(dòng)將這個(gè)值轉(zhuǎn)換成所需要的類型。

1.字符串的不可變性 

  字符串定義了后,會(huì)一直占據(jù)內(nèi)存空間,企鵝該處內(nèi)存空間(棧)不可被重新賦值。

2.短路運(yùn)算

  ||、&& 二元運(yùn)算符,返回參與運(yùn)算的操作數(shù)的原值(原數(shù)據(jù)類型和原數(shù)據(jù)),

  運(yùn)算結(jié)束后,返回導(dǎo)致運(yùn)算結(jié)束的那個(gè)操作數(shù)。

3.三元運(yùn)算符

  code1?code2:code3;   與if-else 不同:

  返回code2或code3的值----code2,code3   都可以空的{}代替;

                   不能寫break,continue。

4.NaN

  NaN !=NaN,

  任何NaN參與的數(shù)學(xué)運(yùn)算,其結(jié)果都是NaN

  有NaN參與的條件表達(dá)式:    比較運(yùn)算符 >/>=/</<=/==/===     運(yùn)算結(jié)果為false

                     !==/!=         運(yùn)算結(jié)果為true

<script>
  var a;
  console.log(Boolean(NaN>=4));
  console.log(Boolean(NaN<4));
  console.log(Boolean(NaN=4));
  console.log(Boolean(NaN==4));
  console.log(Boolean(a=4));
  console.log(NaN);
  console.log(a);
  if(NaN==NaN){
    a = "NaN==NaN";
  }
  var b;
  if(NaN!==NaN){
    b = "NaN!=NaN";
  }
  console.log(a+"\n"+b);
</script>

5. JS 簡(jiǎn)單數(shù)據(jù)類型的轉(zhuǎn)換---特殊情況演示
   數(shù)據(jù):0,“”,false,null,undefined,"123abc"等

<!DOCTYPE html>
<html>
<head lang="en">
  <meta charset="UTF-8">
  <title></title>
  <style>
    div {
      line-height: 24px;
      margin: 0;
      padding: 0;
    }
    .one {
      width: 920px;
      position: absolute;
      left: 50%;
      top: 50%;
      margin-left: -460px;
      margin-top: -240px;
    }
    .all {
      float: left;
      border: 2px solid #000000;
    }
    .all-top {
      font-size: 20px;
      font-weight: bold;
    }
    .all-bottom {
      line-height: 48px;
      font-size: 16px;
    }
    .details {
      float: left;
      border: 2px solid #000000;
      line-height: 24px;
      margin-left: -2px;
    }
    .details:hover {
      position: relative;
      border: 2px solid #ff0000;
    }
    .line-long {
      border-top: 2px solid #000000;
      height: 0;
      width: 908px;
    }
    .line-short {
      border-top: 2px dashed #000000;
      height: 0;
      width: 742px;
      margin-left: 166px;
    }
  </style>
  <script>
    document.write("<div class='one'>");
    function f1() {
      return typeof res[res.length - 1];
    }
    var arr = [0, "", false, null, undefined, NaN, 6.66, -9, "abc124", "-12.23abc23", "qwer", "s s"];
    document.write("<div class='all'><div class='all-top'>" + "&nbsp;&nbsp;原數(shù)據(jù)及類型" + "</br>" + "轉(zhuǎn)換方法&nbsp;&nbsp;</div>" + "<div class='all-bottom'>" + "+" + "</br>" + "Number()" + "</br>" + "parseInt()" + "</br>" + "parseFloat()" + "</br>" + "\"\"" + "</br>" + ".toString" + "</br>" + "String()" + "</br>" + "!!" + "</br>" + "Boolean()" + "</br>" + "</div></div>")
    for (var i = 0; i < arr.length; i++) {
      switch (true) {
        case arr[i] === "":
        {
          var res = ['""'];
          break;
        }
        default :
        {
          var res = [arr[i] + ""];
        }
      }
      res[res.length] = typeof arr[i];
      res[res.length] = +arr[i];
      res[res.length] = f1();
      res[res.length] = Number(arr[i]);
      res[res.length] = f1();
      res[res.length] = parseInt(arr[i]);
      res[res.length] = f1();
      res[res.length] = parseFloat(arr[i]);
      res[res.length] = f1();
      res[res.length] = arr[i] + "";
      res[res.length] = f1();
      if (i == 3 || i == 4) {//null 和undefined沒(méi)有.toString()方法,導(dǎo)致報(bào)錯(cuò)
        res[res.length] = "報(bào)錯(cuò)";
        res[res.length] = "報(bào)錯(cuò)";
      } else {
        res[res.length] = (arr[i]).toString();
        res[res.length] = f1();
      }
      res[res.length] = String(arr[i]);
      res[res.length] = f1();
      res[res.length] = !!arr[i];
      res[res.length] = f1();
      res[res.length] = Boolean(arr[i]);
      res[res.length] = f1();

      var resString = res.join("<br>");
      document.write("<div class='details'>" + resString + "</br>" + "</div>");
    }
    var j = 22;
    for (var i = 0; i < 9; i++) {
      document.write("<div class='line-short' style='margin-top:" + j + "px'></div>")
      document.write("<div class='line-long' style='margin-top:" + j + "px'></div>")
    }
    document.write("</div>");
  </script>
</head>
<body>
</body>
</html>

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