Question

I tried and got the following output:

[object Object]

I am familiar with adding new functions to Object.prototype so that every object in the program inherits that new function. But I am a bit curious to know how this has been implemented. I am guessing Object should of type

{
    name: expression,
    name: expression,
    ...
}

I am also guessing Object.prototype should be the key to one of the Object property and the value of that property is ‘[object Object]’. Now would you please let me know if my understanding is correct?

I am also wondering what is the difference between ‘object’ and ‘Object’ in ‘[object Object]’. I am also wondering if any of the above mentioned objects is related to the one preceding ‘.prototype’. Would you please clarify?

When I tried to print Object.prototype.object, Object.prototype.Object, Object.prototype[object], Object.prototype[Object], and Object.prototype[0], I am always getting undefined. If prototype doesn’t have any property, how did I get [object Object]?

Answer 1

The object is always there and the Object comes from the value of the [[Class]] internal property which is why

Object.prototype.toString.call([]) === "[object Array]";
Object.prototype.toString.call("") === "[object String]";
Object.prototype.toString.call(new Date) === "[object Date]";

It’s described in 15.2.4.2:

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 "]".

Internal properties are a bit confusing. You can read up on them at 8.6.2:

This specification uses various internal properties to define the semantics of object values. These internal properties are not part of the ECMAScript language. They are defined by this specification purely for expository purposes. An implementation of ECMAScript must behave as if it produced and operated upon internal properties in the manner described here. The names of internal properties are enclosed in double square brackets [[ ]].

Incidentally, the Object.prototype.toString.call trick is used by jQuery and other libraries in their isArray functions since the internal property is the same for all frames even though each frame has its own version of the Array builtin.