3-1 string,boolean—>number()只能识别纯数字的字符串

//string-->number 
    var str = "123";
    var s = "123px";
    var a = "";//空串或者全是空格的字符
    console.log(Number(a)); //0
    console.log(Number(str));//123
    console.log(Number(s));//NaN
        *console.log(Number(NaN));//NaN
// boolean-->number
    console.log(Number(true)); //1
    console.log(Number(false)); //0

3-2 string,number—>boolean()

          //number-->Boolean()
        // 特殊：0-->false;
        var b = 0;
        console.log(Boolean(b)); //false
        //string-->Boolean()
        // 特殊："" -->false;
        var d = "";
        console.log(Boolean(d));//false

3-3 Any-string

String();  toString();  +"";
        var a = 123;
        var b = 45 + "";
        console.log(a.toString());//"123"
        console.log(String(a));//"123"
        console.log(b);//"45"

3-4 Any-number

Number(); parseInt(); parseFloat();
        var a = "12d3px";
        var b = "12.34px"
        console.log(parseInt(a));//12
        console.log(parseFloat(b));//12.34

3-5 Any-boolean

boolean();