string,boolean —>number Number(value)

string —> number

// string --> number 只能识别纯数字的字符串
var str = "10";
var s = "hello world"
var z = "2121sgd"
var num = Number(str)
console.log(num)//10
console.log(Number(s))  //NaN 不是一个数字
console.log(Number(z))  //NaN

boolean —> number

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

number,string —>boolean Boolean(value)

**undefined,"",NaN,0 --> false**

3-2-1 number —> boolean

特点： 0:false 非0：true

var a = 0;
var b = -1;
var c = 20;
console.log(Boolean(a))  // false
console.log(Boolean(b))  // true
console.log(Boolean(c))  // true

3-2-2 string —> boolean

特点：空字符串””为:false 其余都为:true

var str = "hello world"
var str1 = " "
var str2 =""
console.log(Boolean(str))  // true
console.log(Boolean(str1)) // true
console.log(Boolean(str2)) // false

任意类型转为number parseInt() parseFloat()

var str = "12px";
var str2 = "12.12121px"
console.log(parseInt(str))//12
console.log(parseFloat(str2))//12.12121

任意类型转为string String() toString()

var a = true 
var str = true+"123" 
console.log(str)//true123
console.log(typeof str)//string
console.log(String(a))//true
console.log(typeof a.toString())//string