JavaScript Object Management
Property Management Methods
// Adding or changing an object property
Object.defineProperty(object, property, descriptor)
// Adding or changing object properties
Object.defineProperties(object, descriptors)
// Accessing a Property
Object.getOwnPropertyDescriptor(object, property)
// Accessing Properties
Object.getOwnPropertyDescriptors(object)
// Returns all properties as an array
Object.getOwnPropertyNames(object)
// Accessing the prototype
Object.getPrototypeOf(object)
JavaScript Object.defineProperty()
The Object.defineProperty() method can be used to:
- Add a new property to an object
- Change property values
- Change property metadata
- Change object getters and setters
Syntax:
Object.defineProperty(object, property, descriptor)
Adding a new Property
This example adds a new property to an object:
Example
// Create an Object:
const person = {
firstName: "John",
lastName :
"Doe",
language : "EN"
};
// Add a Property
Object.defineProperty(person, "year",
{value:"2008"});
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Changing a Property Value
This example changes a property value:
Example
// Create an Object:
const person = {
firstName: "John",
lastName : "Doe",
language : "EN"
};
// Change a Property
Object.defineProperty(person, "language",
{value : "NO"});
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Property Attributes
All properties have a name. In addition they also have a value.
The value is one of the property's attributes.
Other attributes are: enumerable, configurable, and writable.
These attributes define how the property can be accessed (is it readable?, is it writable?)
In JavaScript, all attributes can be read, but an assignment can only change the value attribute (and only if the property is writable).
The Object.defineProperty() method can change the other
attributes (if the property is configurable).
Changing Meta Data
The following property meta data can be changed:
writable : true // Property value can be changed
enumerable : true // Property can be enumerated
configurable : true // Property can be reconfigured
writable : false // Property value can not be changed
enumerable : false // Property cannot be enumerated
configurable : false // Property cannot be reconfigured
Getters and setters can also be changed:
// Defining a getter
get: function() { return language }
// Defining a setter
set: function(value) { language = value }
This example makes language read-only:
Object.defineProperty(person, "language", {writable:false});
This example makes language not enumerable:
Object.defineProperty(person, "language", {enumerable:false});
JavaScript getOwnPropertyNames()
The Object.getOwnPropertyNames() method can:
- List object properties
Syntax
Object.getOwnPropertyNames(object)
List all Object Properties
This example gets all properties of an object:
Example
// Create an Object
const person = {
firstName: "John",
lastName : "Doe",
language : "EN"
};
// Get all Properties
Object.getOwnPropertyNames(person);
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Object.getOwnPropertyNames() will also list properties that are not enumerable:
Example
// Create an Object
const person = {
firstName: "John",
lastName : "Doe",
language : "EN"
};
// Set the language Property not enumerable
Object.defineProperty(person, "language", {enumerable:false});
// Get all Properties
Object.getOwnPropertyNames(person);
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JavaScript Object.keys()
The Object.keys() method can:
- List enumerable object properties
Syntax
Object.keys(object)
List Enumerable Object Properties
This example uses Object.keys() instead of Object.getOwnPropertyNames():
Example
// Create an Object
const person = {
firstName: "John",
lastName : "Doe",
language : "EN"
};
// Change the "language" Property
Object.defineProperty(person, "language", {enumerable:false});
// Get all Enumerable Properties
Object.keys(person);
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The getOwnPropertyNames() method returns all properties.
The Object.keys() method returns all enumerable properties.
If you define object properties without enumerable:false, the two methods will return the same.
Adding Getters and Setters
The Object.defineProperty() method can also be used to add Getters and
Setters:
Example
//Create an object
const person = {firstName:"John", lastName:"Doe"};
// Define a getter
Object.defineProperty(person, "fullName", {
get:
function () {return this.firstName + " " + this.lastName;}
});
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A Counter Example
The getter only reads the value. The setter changes it, and ignores negative numbers.
Example
// Define object
const obj = {_count : 0};
// Define a getter and a setter
Object.defineProperty(obj, "count", {
get : function () {return this._count;},
set : function (value) {
if (value >= 0) {this._count = value;}
}
});
// Play with the counter:
obj.count = 5;
obj.count = obj.count + 1;
obj.count = -3;
let text = obj.count;
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Prototype Properties
JavaScript objects inherit the properties of their prototype.
The delete keyword does not delete inherited properties, but if you delete a
prototype property, it will affect all objects
inherited from the prototype.
