@mhegazy I used to think that abstract classes are just like interfaces but with some members implemented. I know that TypeScript very often works non-intuitively different from other languages, but that is the difference in this specific case comparing to languages that don't have such problems with abstract classes? Angular typescript dependency-injection abstract-class. // use protected property parameter in abstract class // for accessibility from descendants. In JavaScript all class instance properties and methods are public. TypeScript Private Properties. Using TypeScript, we can add private functionality into our classes. What are private properties or methods? A private property of method can only be accessed or called from the class instance itself. Let’s take a look at an example private. 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.
- Typescript Abstract Classes Constructor
- Typescript Abstract Class Vs Interface
- Typescript Abstract Static Property
- Typescript Abstract Property Cannot Be Accessed In The Constructor
Typescript Abstract Classes Constructor
If you are doing traditional OOP with TypeScript, the structural features of TypeScript mightsometimes get in your way. Look at the following class hierachy for instance:
The
FilterItem
abstract class needs to be implemented by other classes. In this example byAFilter
and BFilter
. So far, so good. Classical typing works like you are used to fromJava or C#:When we need the structural information, though, we leave the realms of traditional OOP.Let’s say we want to instantiate new filters based on some token we get from an AJAX call.To make it easier for us to select the filter, we store all possible filters in a map:
The map’s generics are set to a string (for the token from the backend), and everythingthat complements the type signature of
FilterItem
. We use the typeof
keyword hereto be able to add classes to the map, not objects. We want to instantiate them afterwards,after all.So far everything works like you would expect. The problem occurs when you want tofetch a class from the map and create a new object with it.
![Typescript Abstract Property Typescript Abstract Property](https://image.slidesharecdn.com/typescriptdnugpbv2-160425181135/95/typescript-21-638.jpg?cb=1494269221)
What a problem! TypeScript only knows at this point that we get a
FilterItem
back,and we can’t instantiate FilterItem
. Since abstract classes mix type information andactualy language (something that I try to avoid), a possible solution is to move tointerfaces to define the actual type signature, and be able to create properinstances afterwards:Note the
new
keyword. This is a way for TypeScript to define the type signatureof a constructor function.Lots of ?s start appearing now. No matter where you put the
implements IFilter
command, no implementation seems to satisfy our contract:What’s happening here? Seems like neither the implementation, nor theclass itself seem to be able to get all the properties and functionswe’ve defined in our interface declaration. Why?
JavaScript classes are special: They have not only one type we could easilydefine, but two types! The type of the static side, and the type of the instanceside. It might get clearer if we transpile our class to what it was before ES6:a constructor function and a prototype:
Typescript Abstract Class Vs Interface
One type to create the object. One type for the object itself. So let’s splitit up and create two type declarations for it:
The first type
FilterConstructor
is the constructor interface. Here are all static properties,and the constructor function itself. The constructor function returns an instance: IFilter
.IFilter
contains type information of the instance side. All the functions we declare.By splitting this up, our subsequent typings also become a lot clearer:
Typescript Abstract Static Property
- We add
FilterConstructor
s to our map. This means we only can add classes thatprocude the desired objects. - What we want in the end is an instance of
IFilter
. This is what the constructorfunction returns when being called withnew
.
Our code compiles again and we get all the auto completion and tooling we desire.Even better: We are not able to add abstract classes to the map. Because they don’tprocude a valid instance:
Traditional OOP, weaved in into our lovely type system. ✅
Typescript Abstract Property Cannot Be Accessed In The Constructor
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