TypeScript includes the readonly keyword that makes a property as read-only in the class, type or interface.
Prefix
readonly
is used to make a property as read-only. Read-only members can be accessed outside the class, but their value cannot be changed. Since read-only members cannot be changed outside the class, they either need to be initialized at declaration or initialized inside the class constructor. TypeDoc runs the TypeScript compiler and extracts type information from the generated compiler symbols. Therefore you don’t have to include additional metadata within your comments, TypeScript specific elements like classes, enumerations or property.
In the above example, we have the
Employee
class with two properties- empName
and empCode
. Since empCode
is read only, it can be initialized at the time of declaration or in the constructor. TypeScript is the one of the tools people want to learn most, according to a Stack Overflow Survey of 90,000 developers. TypeScript has exploded in popularity, community size, and adoption over the past few years. This blog is a part of my TypeScript series, and the previous ones are: 1. Why program in TypeScript 2. Structural vs nominal typing 3. Getting started with TypeScript classes TypeScript includes the keywords public, protected, and private to control access to the members of a class i.e. Properties or methods. Oct 26, 2018 This blog is a part of my TypeScript series, and the previous ones are: 1. Why program in TypeScript 2. Structural vs nominal typing 3. Getting started with TypeScript classes 4. Access modifiers public, private, and protected My video lessons on TypeScript are here. If you add the abstract keyword to the class declaration, it.
If we try to change the value of
empCode
after the object has been initialized, the compiler shows the following compilation error:error TS2540: Cannot assign to empCode' because it is a constant or a read-only property.
An interface can also have readonly member properties.
As you can see above,
empCode
is readonly, so we can assign a value at the time of creating an object but not after wards.In the same way you can use
Readonly<T>
to create a readonly type, as shown below.In the above example,
emp1
is declared as Readonly<IEmployee>
and so values cannot be changed once initialized. Introduction
TypeScript, like ECMA Script 6, support object-oriented programming using classes. This contrasts with older JavaScript versions, which only supported prototype-based inheritance chain.
The class support in TypeScript is similar to that of languages like Java and C#, in that classes may inherit from other classes, while objects are instantiated as class instances.
Also similar to those languages, TypeScript classes may implement interfaces or make use of generics.
Abstract Classes
Abstract classes are base classes from which other classes can extend. They cannot be instantiated themselves (i.e. you cannot do
new Machine('Konda')
).The two key characteristics of an abstract class in Typescript are:
- They can implement methods of their own.
- They can define methods that inheriting classes must implement.
For this reason, abstract classes can conceptually be considered a combination of an interface and a class.
Accessors
In this example, we modify the 'Simple class' example to allow access to the
speed
property. Typescript accessors allow us to add additional code in getters or setters.Basic Inheritance
This examples shows how to create a very simple subclass of the
Car
class using the extends
keyword. The SelfDrivingCar
class overrides the move()
method and uses the base class implemention using super
.Constructors
In this example we use the
constructor
to declare a public property position
and a protected property speed
in the base class. Beyblade gba rom deutsch download. These properties are called Parameter properties. They let us declare a constructor parameter and a member in one place.One of the best things in TypeScript, is automatic assignment of constructor parameters to the relevant property.
All this code can be resumed in one single constructor:
And both of them will be transpiled from TypeScript (design time and compile time) to JavaScript with same result, but writing significantly less code:
Constructors of derived classes have to call the base class constructor with
super()
.Monkey patch a function into an existing class
Sometimes it's useful to be able to extend a class with new functions. For example let's suppose that a string should be converted to a camel case string. So we need to tell TypeScript, that
String
contains a function called toCamelCase
, which returns a string
.Now we can patch this function into the
String
implementation.If this extension of
String
is loaded, it's usable like this:Simple class
In this example, we declare a simple class
Car
. The class has three members: a private property speed
, a public property position
and a public method move
. Note that each member is public by default. Zee tv serial kumkum bhagya mp3 ringtone download. That's why move()
is public, even if we didn't use the public
keyword.Transpilation
Given a class
SomeClass
, let's see how the TypeScript is transpiled into JavaScript.TypeScript source
Typescript Abstract Static
JavaScript source
When transpiled using TypeScript
v2.2.2
, the output is like so:Observations
Typescript Abstract Class Property
- The modification of the class' prototype is wrapped inside an IIFE.
- Member variables are defined inside the main class
function
. - Static properties are added directly to the class object, whereas instance properties are added to the prototype.