knockout.js - Knockoutjs initialize observableArray with data -

this way trying initialize observablearray data @ time declaration giving error typeerror: this.book not function

here jsfiddle link http://jsfiddle.net/jvqy8/11/

var books = [{book:"harry potter",author:"j.k rowling"},{book:"5 point someone",author:"chetan bhagat"},{book:"i had love story",author:"ravinder singh"}];  var appviewmodel = function() {             this.firstname = ko.observable("amit");             this.books = ko.observablearray(books);             this.updatebook = function() {                 this.book("harry potter , prisoner of azkaban");             }         }; 

when try initialize observable array way this.books = ko.observablearray(books); throwing error typeerror: this.book not function

but moment initialize observable array way this.books = ko.mapping.fromjs(books); working fine.

so can tell me wrong way initialization code this.books = ko.observablearray(books);

could please me answer can understand difference between these 2 approaches below.

1) this.books = ko.mapping.fromjs(books);

2) this.books = ko.observablearray(books);

tell me happens internally when line execute this.books = ko.mapping.fromjs(books);

and happens internally when line execute this.books = ko.observablearray(books);

when use this.books = ko.observablearray(books), thing observable array itself, trying set property (like book) if observable naturally fail.

when using this.books = ko.mapping.fromjs(books) instead, each object in array , each field made observable, why this.book('new title') works.