详解JavaScript对象转原始值

let obj = {
  [Symbol.toPrimitive](hint) {
    switch (hint) {
      case 'number':
        return 123;
      case 'string':
        return 'str';
      case 'default':
        return 'default';
      default:
        throw new Error();
     }
   }
};
2 * obj // 246
3 + obj // '3default'
obj == 'default' // true
String(obj) // 'str'

let a = {
  toString () {
    return '2'
  }
}
console.log(String(a)) // 2

let a = {
  toString () {
    return '2'
  }
}
console.log(a + 'vv')

let a = {
  [Symbol.toPrimitive] (hint) {
    console.log(hint) // string
    return 2
  }
}
console.log(`你是老${a}？`) // 你是老2？

let a = {
  valueOf () {
    return 2
  }
}
console.log(2 / a, a / 2) // 1  1

let a = {
  [Symbol.toPrimitive] (hint) {
    console.log(hint) // number
    return 2
  }
}
console.log(Number(a)) // 2

let a = {
  [Symbol.toPrimitive] (hint) {
    console.log(hint) // number
    return 2
  }
}
console.log(+a) // 2
console.log(-a) // -2

let a = {
  [Symbol.toPrimitive] (hint) {
    console.log(hint) // default
    return 2
  }
}
console.log(1 + a) // 3

let a = {
  [Symbol.toPrimitive] (hint) {
    console.log(hint) // 没有触发
    return false
  }
}
console.log(Boolean(a), a && 123) // true 123