parseInt/類型轉換/字符串

1.pa'rseInt整型

1.1parseInt必須以數字開頭的

var topVal = parseInt("28px");
        console.log(topVal);
        //28

1.2非數字開頭時，控制器打印出來是NaN

var z = parseInt("abc52");
        console.log(z);
        //NaN

1.3兩個參數情況下，16進制

console.log(parseInt("0xf", 16))
        //15

2.parseFloat浮點數

var e = parseInt("12.34.26");
        console.log(e)
        //12
        var d = parseFloat("12.34.26");
        console.log(d);
        //12.34

var b = parseInt("0123abc");
        console.log(b);
        //123
        var dd = parseFloat("0123abc");
        console.log(dd);
        //123
        var ee = Number("0123abc");
        console.log(ee);
        //NaN
        var cc = Number("18");
        console.log(cc);
        //18

注意：當把一個字符串中數字提取出來，最好用parseInt或者parseFloat，前提條件必須是數字開頭（125abc），而不是其他開頭（abc123）；不太推薦Number強制轉換

 

2.轉換為字符串  string與toString

<h2>轉換為字符串類型</h2>
    <script>
        var ids = 65352;
        var idstr = ids.toString();
        console.log(idstr);//65352
        console.log(typeof idstr)//string
        console.log(String(ids))//65352
    </script>

3.轉為布爾Boolean

<h2>轉換為布爾類型</h2>
    <script>
        var ids = true;
        var idstr = false;
        console.log(typeof ids);//boolean
        console.log(typeof idstr)//boolean
    </script>

3.1類型轉換 

（1）除0之外的所有數字，轉換為布爾型都為true

（2）除""之外的所有字符，都轉為布爾型都為true

（3）null和undefined轉換為布爾型為false

<h2>除0之外的所有數字，轉換為布爾型都為true</h2>
    <script>
        var x = 0;
        var y = 1;
        console.log(Boolean(x));//false
        console.log(Boolean(y))//true
    </script>

<h2>除""之外的所有字符，都轉為布爾型都為true</h2>
    <script>
        var stringWords = "welcome";
        var stringKong = "";//空的
        var kongge = " ";
        console.log(Boolean(stringKong));//false
        console.log(Boolean(stringWords))//true
        console.log(Boolean(kongge))//true  空格布爾型也是true
    </script>

<h2>null和undefined轉換為布爾型為false</h2>
    <script>
        var y;
        var x = null;
        console.log(Boolean(y))//false
        console.log(Boolean(x))//false
    </script>