Javascript object programming and object representation

Question

Scenario

I was toying with some Javascript object programming, when I wrote down the following code (demo here):

    var o = {
        "key":1,
        "value": 1234,
        toString: function(){
            var res = this.key + ";" + this.value;
            return res;
        }
    };

    var p = {
        "key":2,
        "value": 5678
    };

    document.write(o);
    document.write("</br>");
    document.write(p);

As you can see, having a toString() method let the o object to be displayed without calling the toString() method, while the p object is displayed as... as what?

QUESTIONS

Is the toString literal a kind of default name for objects representation? Or what else?
What does the [object Object] notation mean?

Thanks a lot to everybody. You gave me references and good points. I started using Javascript a long time ago, before famous frameworks let everybody know the power of this scripting language. Now I am fascinated by oop capabilities of the ECMA-262: the old good Javascript. — Alberto De Caro
– Alberto De Caro, Commented Sep 28, 2012 at 8:07

Bergi · Accepted Answer · 2012-09-27 14:44:08Z

Is the toString literal a kind of default name for objects representation? Or what else?

It is a function on the toString property of your object.

In Javascript, when an object is casted to a string (EcmaScript §9.8: ToString, e.g. when used in a string concatenation, or in alert() or document.write()) it's toString method is called (via ToPrimitive and DefaultValue - §8.12.8).

Often, you specify that on the prototype of your custom objects, and also all builtin types have one (e.g. Array.prototype.toString, which joins the values with commata). You have explicitly set it in your object literal.

What does the [object Object] notation mean?

This is the result of the default toString implementation of objects, ie. for all instances of Object. EcmaScript §15.2.4.2 specifies Object.prototype.toString to return [object+class+], and the internal [[class]] of plain objects is "Object".

Hogan · Accepted Answer · 2012-09-27 14:29:59Z

1

toString is not a literal, it is a function and is defined for all objects. Remember functions are "first class" in javascript.

The [object Object] is the return value of the browser's default toString() for an object.

Here is Mozilla's excellent documentation:

https://developer.mozilla.org/en-US/docs/JavaScript/Reference/Global_Objects/Object/toString

answered Sep 27, 2012 at 14:29

Hogan

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2 Comments

Bergi Over a year ago

No, it does not look different

Alberto De Caro Over a year ago

+1 one for the reference and the remark about the "first class"!

Ja͢ck · Accepted Answer · 2012-09-27 14:36:48Z

0

Every object in JavaScript has a toString() method, which gets called implicitly when the object is used in a string context, such as with document.write(o) but also when you write o + ' another string'.

The method returns '[object Object]' by default which is not very useful, but it can be overridden to return anything you like; Objects like Numeric, String or Date do this for instance.

answered Sep 27, 2012 at 14:36

Ja͢ck

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Comments

Philipp · Accepted Answer · 2012-09-27 14:40:54Z

0

When an object is sent to write, write internally calls the toString method of the object which might (like in o) or might not (like in p) be overridden.
In the default implementation of toString, the first object defines that it is an object (in contrary to an array or primitive) and the second Object defines that it is an object of type Object. Some build-in object types which don't have a meaningful string represenation (like DOM nodes, for example), return their type name instead of Object. When you want your own objects to output their type name when passed to write, you have to override toString.

edited Sep 27, 2012 at 14:40

answered Sep 27, 2012 at 14:33

Philipp

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Comments

Mitch Satchwell · Accepted Answer · 2012-09-27 14:35:46Z

-1

'[object Object]' is returned by the default toString method which is inherited by every object descended from Object.

answered Sep 27, 2012 at 14:35

Mitch Satchwell

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