why Object.prototype.toString return [object Object] [closed]

Answer 1

According to ECMAScript Language Specification:

15.2.4.2 Object.prototype.toString ( ) When the toString method is called, the following steps are taken:

1. If the this value is undefined, return "[object Undefined]".
2. If the this value is null, return "[object Null]".
3. Let O be the result of calling ToObject passing the this value as the argument.
4. Let class be the value of the [[Class]] internal property of O.
5. Return the String value that is the result of concatenating the three Strings "[object ", class, and "]".

The language is designed like this. You'd have to ask Brendan Eich, or TC39, I guess.