Properties are members of an object. Each named property is a pair of (name, descriptor). The name is a string that allows access (using the dot notation object.propertyName or the square brackets notation object['propertyName']). The descriptor is a record of fields defining the bevahiour of the property when it is accessed (what happens to the property and what is the value returned from accessing it). By and large, a property associates a name to a behaviour (we can think of the behaviour as a black box).

There are two types of named properties:

1. data property: the property’s name is associated with a value.
2. accessor property: the property’s name is associated with one or two accessor functions.

Demonstration:

obj.propertyName1 = 5; //translates behind the scenes into
                       //either assigning 5 to the value field* if it is a data property
                //or calling the set function with the parameter 5 if accessor property

// *actually whether an assignment would take place in the case of a data property
// also depends on the presence and value of the writable field - on that later on

The property’s type is determined by its descriptor’s fields, and a property cannot be of both types.

Data descriptors -

• Required fields: value or writable or both
• Optional fields:configurable,enumerable

Sample:

{
   value: 10,
   writable: true;
}

Accessor descriptors -

• Required fields: get or set or both
• Optional fields: configurable, enumerable

Sample:

{
    get: function () {
        return 10;
    },
    enumerable: true
}

meaning of fields and their defaults

configurable,enumerable and writable:

• These keys all default to false.