输出对象先toString再valueOf

先调用toString,若返回基础类型,则输出

    var test = {
        toString:function () {
            console.log('toString');
            return 1;
        },
        valueOf:function () {
            console.log('valueOf');
            return 2;
        }
    }
    alert(test);   
    // toString
    // 弹出1

若toString返回的不是基础类型,则继续调用valueOf

    var test = {
        toString:function () {
            console.log('toString');
            return {};
        },
        valueOf:function () {
            console.log('valueOf');
            return 2;
        }
    }
    alert(test);   
    // toString
    // valueOf
    // 弹出2

若两者都返回非基础类型值,则抛出异常

    var test = {
        toString:function () {
            console.log('toString');
            return {};
        },
        valueOf:function () {
            console.log('valueOf');
            return {};
        }
    }
    alert(test);   
    // toString
    // valueOf
    // Uncaught TypeError: Cannot convert object to primitive value

参与运算先valueOf再toString

先调用valueOf,若返回基础类型,则进行运算

    var test = {
        toString:function () {
            console.log('toString');
            return 1;
        },
        valueOf:function () {
            console.log('valueOf');
            return 2;
        }
    }
    alert(test + 1);   
    // valueOf
    // 弹出3

若valueOf返回非基础类型,则继续调用toString

    var test = {
        toString:function () {
            console.log('toString');
            return 1;
        },
        valueOf:function () {
            console.log('valueOf');
            return {};
        }
    }
    alert(test + 1);  
    // valueOf 
    // toString
    // 弹出2

若两者返回均为非基础类型,则抛出异常

    var test = {
        toString:function () {
            console.log('toString');
            return {};
        },
        valueOf:function () {
            console.log('valueOf');
            return {};
        }
    }
    alert(test + 1);   
    // valueOf
    // toString
    // Uncaught TypeError: Cannot convert object to primitive value

应用

• == 比较
  若对象和字符串或者数值比较,则先要将对象先转换成基础类型再进行比较

      var a = {a:1};
      a.valueOf = a.toString = function () {
          return 1;
      }
      a == '1' // true
  

• add(2)(3)(4)

      function add(arg){
          var sum = arg,
              result = function (b) {
                  sum += b;
                  return result;
              }
          result.valueOf = result.toString = function(){
              return sum;
          }
          return result;
      }
      add(2)(3)(4)