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JavaScript数据类型转换的注意事项

时间:2021-07-01 10:21:17 帮助过:21人阅读

1.字符串的不可变性 

  字符串定义了后,会一直占据内存空间,企鹅该处内存空间(栈)不可被重新赋值。

2.短路运算

  ||、&& 二元运算符,返回参与运算的操作数的原值(原数据类型和原数据),

  运算结束后,返回导致运算结束的那个操作数。

3.三元运算符

  code1?code2:code3;   与if-else 不同:

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

                   不能写break,continue。

4.NaN

  NaN !=NaN,

  任何NaN参与的数学运算,其结果都是NaN

  有NaN参与的条件表达式:    比较运算符 >/>=/</<=/==/===     运算结果为false

                     !==/!=         运算结果为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 简单数据类型的转换---特殊情况演示
   数据: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'>" + "  原数据及类型" + "</br>" + "转换方法  </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没有.toString()方法,导致报错
        res[res.length] = "报错";
        res[res.length] = "报错";
      } 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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