"for ... in" and "for each ... in" Statements

This section provides a quick description of two special loop statements for iterating properties and methods of object. A tutorial example is provided to show the extra properties and methods of two objects of the same type.

JavaScript supports two special loop statements that allows you to iterate all properties and methods in an object:

1. A "for ... in" statement has the following syntax:

   for (var item in object_name) {
      ... statements with object_name[item] ... 
   }

In this "for ... in" statement, the embedded statements will be executed for each properties and methods. Variable "item" will contain the current property or method name. Expression "object_name[item]" uses the associate array format to retrieve the property value.

2. A "for each ... in" statement has the following syntax:

   for each (var value in object_name) {
      ... statements with value ... 
   }

In this "for each ... in" statement, the embedded statements will be executed for each properties and methods. Variable "value" will contain the value of the current property or the function of the current method.

Here is a tutorial example that use both "for ... in" and "for each ... in" statements to loop through all properties and methods:

<html>
<!-- Prototype_Property_Method_Loop.html
   Copyright (c) 2013 by Dr. Herong Yang, herongyang.com
-->
<head>
<title>Adding Additional Properties and Methods to Objects</title>
</head>
<body>
<pre>
<script type="text/javascript">

// Defining the constructor function for a new object type
function Book(title, author) {
   this.title = title;
   this.author = author;
}

// Defining a function to be used as a method
function getDescription() {
   return "\""+this.title+"\" by "+this.author;
}

   // Creating an object of "Book"
   var myBook = new Book("JavaScript Tutorials", "Herong Yang");

   // Adding additional property and method to one object
   myBook.price = 99.99;
   myBook.getDesc = getDescription;

   // Showing object properties and methods
   showObject(myBook, "myBook");

function showObject(object, name) {
   document.writeln("\nShowing object \""+name+"\"");
   document.writeln("   Property/method names and values:");
   for (var item in object) {
      document.writeln("      "+item+": "+object[item]);
   }

   document.writeln("\nShowing object \""+name+"\"");
   document.writeln("   Property/method values:");
   for each (var v in object) {
      document.writeln("      "+v);
   }
}
</script>
</pre>
</body>
</html>

The output of this tutorial example is printed below:

Showing object "myBook"
   Property/method names and values:
      title: JavaScript Tutorials
      author: Herong Yang
      price: 99.99
      getDesc: function getDescription() {
    return "\"" + this.title + "\" by " + this.author;
}

Showing object "myBook"
   Property/method values:
      JavaScript Tutorials
      Herong Yang
      99.99
      function getDescription() {
    return "\"" + this.title + "\" by " + this.author;
}

Note that the "for each ... in" statement has been deprecated. Do not use it anymore.

Table of Contents

 About This Book

 Introduction to JavaScript

 ECMAScript Language Specification and JavaScript Dialects

 Data Types, Variables and Expressions

 Flow Control Statements

 Creating, Accessing, and Manipulating Arrays

 Defining and Calling Functions

 Web Browser Supporting JavaScript

 Server-Side and Client-Side Web Scripting

 Introduction to Objects

Defining Your Own Object Types

 Prototype-Dased Object-Oriented Programming Style

 Prototype-Based Programming Features in JavaScript

 Defining Object Constructor Functions

 Adding Properties and Methods to Objects

"for ... in" and "for each ... in" Statements

 "prototype" Property of the Constructor Function Object

 "instanceof" Operator - Determining Object Type

 "typeof" Operator and Data Types

 Inheritance of Properties and Methods through the Prototype Object Chain

 'jrunscript' - JavaScript Shell Command from JDK

 Using Functions as "Function" Objects

 Introduction to Built-in Object Types

 W3C's Document Object Model (DOM) Specifications

 References

 PDF Printing Version