A new object is created using this syntax:
var obj = new Object();. If there are no arguments, the parentheses may be omitted, although Best Practice is to include them. This statement is known as an 'instance of the object obj'.
An object instance has these properties and methods:
Objects use data (properties) and functions (methods) in a composite called a class. When a new object is created with this class it is called an instance of the class.
To create a class, write a function (constructor) with the class name. This function takes arguments, and can create the properties and methods for the object created when an instance is called.
this refers to the instance created each time the function
memberEmail is called, allowing the function to be called any number of times.
The call to a second function
listMemberEmail within the
memberEmail class makes it a method of the class. Functions do not necessarily have to be defined within the class constructor.
To create an instance of a class, first create an empty object:
friend = new memberEmail(), then populate it:
An instance of the new object could also be created by inputting all the arguments in one go:
friend = new memberEmail("Freddie", "McGurty", "firstname.lastname@example.org")
To use the object: e.g.
document.write(friend.firstname + ' ' + friend.surname + ' has email: ' + friend.email)
Often it is necessary to check whether a variable is of a certain type. Consider this function:
If either of the two arguments, a and b, is not numerical, an error will occur.
The function isNaN (is Not a Number) can be used to understand if the object is numerical or not.
Therefore, in our example we can avoid errors:
All well and good for numbers. But what about other objects?
TYPEOF operator is used to reveal a variable's basic type (boolean, sting, numerical, undefined, function).
Here are some examples:
There are, however, some situations in which this operator has an unexpected behaviour:
In general, the best method is the
instanceof operator, in the form
a instanceof b, returns true if a is an instance of b, otherwise false.
Returning to the last examples of the
Unfortunately, however, this method also has its little problems. It does not function with an Array from another frame (i.e. from an Iframe).
So, how is possible to create a function which is reliable, checking whether an object is an Array or not?
One solution is surprisingly simple: the
toString method, which gives the type of object in the form of a string:
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Translation by Transalpine Technical Translations.
Content © Andrew Bone. All rights reserved. Created : April 5, 2014 Last updated :March 10, 2016
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