1. 强制转换 number,string,boolean

1-1 string，boolean —> number

var str = 'w';
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
// 1.能够识别纯数字的字符串
// 2.""," " 识别成0
// 3.NaN

1-2 boolean—> number

// true = 1; false = 0;
var a = true;
var b = false;
console.log(Number(a));
console.log(Number(b));

1-3 string,number —-> boolean

// string,number ---> boolean
// Boolean();
// string --> boolean   特点:'' 没有空格的空字符串为false，其他都为true
var str = 'good'
var s = ' '
var t = ''
console.log(Boolean(str));
console.log(Boolean(s));
console.log(Boolean(t));

1-4 number —> boolean

// number --> boolean
// tips:是只有0为false，其他情况都为true
var num1 = 0
var n = -1
var m = 1
console.log(Boolean(num));
console.log(Boolean(n));
console.log(Boolean(m));

1-5 number，boolean —> string

number，boolean --> string
// String();
var num1 = 10
var t = true
console.log(String(num1));
console.log(String(t));

1-6 转化结果为:false

// 转化结果为:false
// number -- 0
// string -- ""
// unfefined
// NaN