4-1 string,boolean —>number Number(value)

4-1-1 string —> number

Number(“ “) Number(“”) Number(null) => 输出 0

// string --> number 只能识别纯数字的字符串
// Number(" ")  Number("")  Number(null)  => 输出 0
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

4-1-2 boolean —> number

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

4-2 number,string —>boolean Boolean(value)

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

  4-2-1 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

  4-2-2 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

  4-3 任意类型转为number parseInt() parseFloat()

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

  4-4 任意类型转为string String() toString()

  var a = true 
  var str = true+"" 
  console.log(typeof str)
  console.log(typeof String(a))
  console.log(typeof a.toString())