JS类型转换

ToPrimitive：数据类型转化为原始值

function ToPrimitive(target,type){ // 将目标对象转换为指定类型原始值[[primitive]]
  if(target == [[primitive]]) return target

  if(type == String){
    if(target.toString() == [[primitive]]) return target.toString()
    if(target.valueOf() == [[primitive]]) return target.valueOf()
  }
	
  if(type == Number){
    if(target.valueOf() == [[primitive]]) return target.valueOf() // object.valueOf()通常返回自身
    if(target.toString() == [[primitive]]) return target.toString()
  }
  
  else{ // type未传入时优先valueOf()
    if(target.valueOf() == [[primitive]]) return target.valueOf()
    if(target.toString() == [[primitive]]) return target.toString()
  }
}

toString和toNumber：数据类型转化为String or Number

toString(target) == ToPrimitive(target,String)
toNumber(target) == ToPrimitive(target,Number)

+操作

计算 val1 + val2:

if(typeof ToPrimitive(val1) == 'string' || typeof ToPrimitive(val2) == 'string' ) 
  return toString(val1) + toString(val2)
else 
  return toNumber(val1) + toNumber(val2) 

==判断

判断 x == y

let typeX = typeof x, typeY = typeof y

// 类型相同
if(typeX === typeY ){ 
  if(x === null||x === undefined) return true 
  if(typeX === 'String' || typeX === 'Boolean' || typeX === 'Object') return x === y
  if(typeX === 'Number'){
  if(isNaN(x)||isNaN(y)) return false // 众所周知NaN不等于任何，包括自身
    if(x === +0 && y === -0 || x === -0 && y === +0) return true
    return x === y
  }	
}

// 类型不同
// 1.众所周知 null == undefined
if((x===null&&y===undefined) || (x===undefined&&null)) return true 

// 2.二者有一个为number，一个为string
if(typeX === 'Number'&&typeY === 'String') return x == toNumber(y)
if(typeX === 'String'&&typeY === 'Number') return y == toNumber(x)

// 3.二者有一个为boolean
if(typeX === 'Boolean')  toNumber(x) == y
if(typeY === 'Boolean')  toNumber(y) == x

// 4.二者有一个为Object，另一个为number或string
if(typeX === 'Object')  ToPrimitive(x) == y
if(typeY === 'Object')  ToPrimitive(y) == x

参考

1.JS类型转换上 (opens new window)

2.JS类型转换下 (opens new window)