Considerations for JavaScript Data Type Conversion

2021-07-06 09:59:17
OfStack

1. String immutability

After the string is defined, it will occupy the memory space directly, and the memory space (stack) of penguin cannot be re-assigned.

2. Short circuit operation

, & & 2-yuan operator, which returns the original value (original data type and original data) of the operands participating in the operation.

After the operation ends, the operand that caused the operation to end is returned.

3.3 yuan operator

code1? code2: code3; Unlike if-else:

Returns the value of code2 or code3--code2, code3 can all be replaced by an empty {};

You cannot write break, continue.

4.NaN

NaN! = NaN,

Any mathematical operation that NaN participates in, the result is NaN

Conditional expressions involving NaN: Comparison operators > / > =/ < / < =/==/=== The result of the operation is false

! = =/! = The result of the operation is 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 simple data type conversion-special case demonstration
Data: 0, "", false, null, undefined, "123abc", etc


<!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; Original data and type " + "</br>" + " Conversion method &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  And undefined No .toString() Method, causing an error to be reported 
        res[res.length] = " Report an error ";
        res[res.length] = " Report an error ";
      } 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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