輸出對(duì)象先toString再valueOf

先調(diào)用toString,若返回基礎(chǔ)類型,則輸出

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

若toString返回的不是基礎(chǔ)類型,則繼續(xù)調(diào)用valueOf

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

若兩者都返回非基礎(chǔ)類型值,則拋出異常

    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

參與運(yùn)算先valueOf再toString

先調(diào)用valueOf,若返回基礎(chǔ)類型,則進(jìn)行運(yùn)算

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

若valueOf返回非基礎(chǔ)類型,則繼續(xù)調(diào)用toString

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

若兩者返回均為非基礎(chǔ)類型,則拋出異常

    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

應(yīng)用

• == 比較
  若對(duì)象和字符串或者數(shù)值比較,則先要將對(duì)象先轉(zhuǎn)換成基礎(chǔ)類型再進(jìn)行比較

      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)