Javascript empty object

Question

This is an example of code, from a book called JavaScript: The Definitive Guide, 6th Edition, that I do not understand.

He is talking about Objects.

var book ={
    topic: "javascript",
    fat: true
};
book.topic              => "javascript"
book.["fat"]            => True
book.author="flanagan"; // creates new property
book.contents= {};      // empty object*

So what I don't understand is the last part. Is he adding in a new property called "contents" that is empty? Because he is calling it an object and it's confusing me.

No, he is adding a new property called contents and is assigning it a new Object with no properties. — Mr. Polywhirl
– Mr. Polywhirl, Commented Nov 1, 2013 at 9:03
book.["fat"] will give you SyntaxError. Use either dot or square brackets but not both of them simultaneously. — aga
– aga, Commented Nov 1, 2013 at 9:04

David Ewen · Accepted Answer · 2013-11-01 09:01:33Z

2

Yes {} is an empty object in javascript which is being assigned to the contents property of the book object.

Here you can see that we can use functions defined globaly on object such as toString()

answered Nov 1, 2013 at 9:01

David Ewen

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Comments

benzonico · Accepted Answer · 2013-11-01 09:02:30Z

1

Take a look at the way the book variable is defined. It is an Object declaration. In the last line, we add a property named contents and assign it an object declared exactly the same way as book except that this one has no property and therefore is an empty object : {}

answered Nov 1, 2013 at 9:02

benzonico

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Chris Pickford · Accepted Answer · 2013-11-01 09:02:37Z

1

The curly bracket { } notation denotes an object literal.

The fact that there is nothing between them means that a new object reference pointing to a blank object (inheriting from the base object) is stored in the book.contents property.

answered Nov 1, 2013 at 9:02

Chris Pickford

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Ashwani · Accepted Answer · 2013-11-01 09:06:11Z

1

{} create an empty object. Means now this object can have new property same as defined by book.author="flanagan";

// Both are same
book.contents = new Object();
book.contents = { } ;

answered Nov 1, 2013 at 9:06

Ashwani

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Saeed Alizadeh · Accepted Answer · 2013-11-01 09:13:33Z

1

In JavaScript you can create an object with {} notation, called object literal and you can add what ever you want to this object in future by dot even a function , in your example in first step:

var book ={topic: "javascript",fat: true};

book object is created with two property topic and fat and then this object is extended with author as String and contents as an inner object, as I said you can use it for creation empty objects (var t={} //example); and if you use

typeof book.contents // returns "object"

answered Nov 1, 2013 at 9:13

Saeed Alizadeh

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Ian Roberts · Accepted Answer · 2013-11-01 09:01:46Z

0

It's creating a new property of the original book object, and setting the value of that property to a new empty object.

answered Nov 1, 2013 at 9:01

Ian Roberts

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Mark Walters · Accepted Answer · 2013-11-01 09:02:54Z

0

He's adding a new property called contents of the object book which itself is an empty object.

Here is a structure of what it would look like when he's made his changes to help you visualise it.

var book = {
    topic: "javascript",
    fat: true,
    author : "flanagan",
    contents : {}
};

answered Nov 1, 2013 at 9:02

Mark Walters

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2 Comments

Tunafish Ha Over a year ago

So is there another way to say that a property has no value? If one just typed "contents" instead of "contents : {}"

Mark Walters Over a year ago

You can assign object properties in two ways. book["contents] = or book.contents = . The value you assign to it can differ from a function to an empty object or array to a string or number. book.contents = "" now book.contents is a blank string.

kundan Karn · Accepted Answer · 2013-11-01 09:16:32Z

<script type="text/javascript">

        var book ={
            topic: "javascript",
            fat: true
        };


        book.topic = "javascript";
        book.fat = true;
        book.author = "flanagan"; // creates new property
        book.contents = {};      // empty object*
        //The curly bracket { } denotes here an object.
        //will add a property contents and will contain an empty object

        alert(book.contents);   //it will display [object Object]

        book.contents.name = 'Success Stories';
        //adding propertis to the object contents
        alert(book.contents.name);

        /*
        you can consider it as object inside object

        objectOuter {
            property1: value1
            property2: value2
            property3: objectInner {
                property1: value1
                property2: value2
            }
        }
        */       

    </script>

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