2-1 基本数据类型强制转化

1. string，boolean—>number number()

            只能识别纯数字字符串，其他字符输出  Nan

 string转化number
                <script>
            /* string-->number   number()
                只能识别纯数字字符串，其他字符输出  Nan
            */
            var str = "3"
            var num = Number(str);
            console.log(typeof num);
            console.log(Number("3.0"));
            console.log(Number("3.00"));
            console.log(Number("3px")); //nan  不是一个数字的意思
        </script>

 boolean转化number
    <script>
        var a = true  // 输出1
        var b = false // 输出0
        console.log(Number (a))   //  1
        console.log(Number (b))   //  0
    </script>

2. Number，string—>boolean

       1.string-->boolean<br />         特点：除了  ""  空字符-->输出为false，其他都为true。

        var a ="good"
        var b = " "
        var c = ""
        console.log(Boolean (a))
        console.log(Boolean (b))
        console.log(Boolean (c))

       2.Number-->boolean<br />        特点：除了 0-->输出为false，其他都为true。

        var d =0
        var e =1
        var f =1
        console.log(Boolean (d))
        console.log(Boolean (e))
        console.log(Boolean (f))

3. Number，boolean—>string

    <script>
        //String()
        // tostring()
        var num = 10
        var b = true
        console.log(String (num))
        console.log(typeof b.toString() )
    </script>

4. false

    <script>
        //String()
        // tostring()
        var num = 10
        var b = true
        console.log(String (num))
        console.log(typeof b.toString() )
    </script>