JavaScript Objects Descriptors and Named Properties
Example
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:
- data property: the property's name is associated with a value.
- 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:
valueorwritableor both - Optional fields:
configurable,enumerable
Sample:
{
value: 10,
writable: true;
}
Accessor descriptors -
- Required fields:
getorsetor 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. configurableistrueif and only if the type of this property descriptor may be changed and if the property may be deleted from the corresponding object.enumerableistrueif and only if this property shows up during enumeration of the properties on the corresponding object.writableistrueif and only if the value associated with the property may be changed with an assignment operator.
get and set:
- These keys default to
undefined. getis a function which serves as a getter for the property, orundefinedif there is no getter. The function return will be used as the value of the property.setis a function which serves as a setter for the property, orundefinedif there is no setter. The function will receive as only argument the new value being assigned to the property.
value:
- This key defaults to
undefined. - The value associated with the property. Can be any valid JavaScript value (number, object, function, etc).
Example:
var obj = {propertyName1: 1}; //the pair is actually ('propertyName1', {value:1,
// writable:true,
// enumerable:true,
// configurable:true})
Object.defineProperty(obj, 'propertyName2', {get: function() {
console.log('this will be logged ' +
'every time propertyName2 is accessed to get its value');
},
set: function() {
console.log('and this will be logged ' +
'every time propertyName2\'s value is tried to be set')
//will be treated like it has enumerable:false, configurable:false
}});
//propertyName1 is the name of obj's data property
//and propertyName2 is the name of its accessor property
obj.propertyName1 = 3;
console.log(obj.propertyName1); //3
obj.propertyName2 = 3; //and this will be logged every time propertyName2's value is tried to be set
console.log(obj.propertyName2); //this will be logged every time propertyName2 is accessed to get its value
