5.强制转换

5.1 string—>number

         var str = "123";
         var num  = Number(str);
         var s = "hello word";
         var b = "21ab";
//只能识别纯数字的字符串
         console.log(str); //123
         console.log(num); //123 
         console.log(typeof num); //number
//识别其它类型字符串会NaN
         console.log(Number (s)); //NaN
         console.log(Number (b)); //NaN

        var str = "12px";
        var s = "12.34px";
        /* 
           Number();
           parseInt
           parseFloat
           将string-->number有以上三个方法
         */
        console.log(parseInt(str))
        console.log(parseInt(s))
        console.log(parseFloat(s))
        console.log(Number("12.343"))

5.2 boolean—>number

        var t =true;
        var f =false;
        console.log(Number(t));//1
        console.log(Number(f));//0

5.3 string number—>boolean

//string——>boolean
        var str = "hello world";
        var num = "12";
        console.log(Boolean(str)) //true
        console.log(Boolean(num)) //true
        console.log(Boolean(""))  //false 只有字符串为""（空串）时false
        console.log(Boolean(" ")) //true
//number——>boolean
        var a = 10;
        var b = 0;
        var c = -1;
        console.log(Boolean(a)) //true
        console.log(Boolean(b)) //false 只有0为false
        console.log(Boolean(c)) //true

5.4 任意类型—>string String() toString()

        var num  = 10;
        var t = true;
        console.log(String(num))
        console.log(String(t))
        console.log(num.toString())

5.5 任意类型—>number parseInt () parseFloa()

        var str = "12px";
        var s = "12.34px";
        console.log(parseInt(str)) // 可以去掉末尾单位 px
        console.log(parseInt(s))
        console.log(parseFloat(s)) // 可以去掉末尾单位 px
        console.log(Number("12.343"))