JavaScript
Object Properties
Properties
are the most important part of any JavaScript object.
JavaScript
Properties
Properties
are the values associated with a JavaScript object.
A JavaScript
object is a collection of unordered properties.
Properties can
usually be changed, added, and deleted, but some are read only.
Accessing
JavaScript Properties
The syntax
for accessing the property of an object is:
objectName.property // person.age
or
objectName["property"] // person["age"]
or
objectName[expression] // x = "age"; person[x]
The
expression must evaluate to a property name.
Example
1
person.firstname
+ " is " + person.age + " years old.";
»
Example
2
person["firstname"]
+ " is " + person["age"] + " years old.";
»
JavaScript
for...in Loop
The
JavaScript for...in statement loops through the properties of an object.
Syntax
for
(variable in object) {
code to be executed
}
The block of
code inside of the for...in loop will be executed once for each property.
Looping
through the properties of an object:
Example
var
person = {fname:"John", lname:"Doe", age:25};
for
(x in person) {
txt += person[x];
}
»
Adding New
Properties
You can add
new properties to an existing object by simply giving it a value.
Assume that
the person object already exists - you can then give it new properties:
Example
person.nationality
= "English";
»
You cannot
use reserved words for property (or method) names. JavaScript naming rules
apply.
Deleting
Properties
The delete
keyword deletes a property from an object:
Example
var
person = {firstName:"John", lastName:"Doe", age:50,
eyeColor:"blue"};
delete
person.age; // or delete
person["age"];
»
The delete
keyword deletes both the value of the property and the property itself.
After
deletion, the property cannot be used before it is added back again.
The delete
operator is designed to be used on object properties. It has no effect on
variables or functions.
The delete
operator should not be used on predefined JavaScript object properties. It can
crash your application.
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 only the value attribute can be
changed (and only if the property is writable).
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.
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