Javascript expando objects

What are expando objects in javascripts?

For what purpose we need this ? Any complete example will be appreciated

I found 1 article here Javascript: The red-headed stepchild of web development

Well, in javascript, any object is an expando object. What it means is, as the article covers, that whenever you try to access a property¹ it will automatically be created.

var myObj = {}; // completely empty object
myObj.myProp = 'value';

The moment you assign myProp a value, the property myProp is dynamically created, eventhough it didn't exist before. In a lot of other languages, such as C#, this is not normally possible (actually C# has just enabled expando object support as well, but that's besides the point). To access a property in a normal class in C#, you need to specify in the class that it does indeed have this property.

¹ Not quite correct. See npup's comment below for clarification.

Everything except primitive types(string, number,boolean) are objects and support Key:values structure. properties(keys) can be accessed and set using the dot notation as well as the square brackets.

var myObj = {};   
myObj.myProp1 = 'value1'; //works, an expando property   
myObj[myProp2] = 'value2'; // doesn't work, myProp2 is an undefined name.
myObj['myProp2'] = 'value2'; // works  , an expando property   
myObj[2010]= 'value'; //note the key is number, still works, an expando property??   
myObj.2010 = 'value'; // FAILS. to use dot notation, key must be a string