obj+""≠obj.toString()≠String(obj)っていう話 ref: http://qiita.com/items/8d80ca107555489710ea

file0.txt

 // obj.toString()
 str = obj+"";

file2.txt

"toString"
"valueOf"
"toString"

file4.txt

"obj1 valueOf"
"obj2 toString"
TypeError : .... 

file5.txt

Object.prototype.valueOf = function(){
    return this;
};

file6.txt

var
obj1 = {
    toString : function(){
        return {};
    },
    valueOf : function(){
        rettrn "valueOf";
    },
},
obj2 = {
    toString : function(){
        return {};
    },
    valueOf : function(){
        return {};
    },
};

console.log(String(obj1)); //=> "valueOf"
console.log(String(obj2)); //TypeError!

file7.txt

 str = String(obj.toString());

testTransString1.js

var obj = {
    toString : function(){
        return "toString";
    },
    valueOf : function(){
        return "valueOf";
     },
};

console.log(obj.toString()); //=> "toString"
console.log(obj+""); //=> ???
console.log(String(obj)); // ???

testTransString2.js

 var
 obj1 = {
     toString : function(){
         return "obj1 toString";
     },
     valueOf : function(){
         return "obj1 valueOf";
     },
},
obj2 = {
    toString : function(){
       return "obj2 toString";
    },
    valueOf : function(){
        return {};
    },
},
obj3 = {
    toString : function(){
        return {};
    },
    valueOf : function(){
        return {};
    },
};

[obj1,obj2,obj3].forEach(function(obj){
    console.log(obj+"");
});