Partial classes

[disshishkov]

Add support of partial classes. Mixins is not the same, because it's run-time realization. Need compile realization, where partial classes will be combine into one before converting typescript to javascript.

//FileA.ts
partial class ClassA
{      
    constructor(public name: string) {}
    public sayHello(): string { return "hi!"; }
}

//FileB.ts
partial class ClassA
{
   public sayBye(): string { return "by!"; }
}

will be:

partial class ClassA
{      
    constructor(public name: string) {}
    public sayHello(): string { return "hi!"; }
    public sayBye(): string { return "by!"; }
}

[Davidhanson90]

I think you employ "partial" on modules too and help resolve this issue raised here.

#447

[basarat]

@disshishkov perhaps extension methods be sufficient : #9

[RyanCavanaugh]

What kind of situations do you want to use this for, and how do existing solutions fall short of that?

Note that this was implemented in C# to support WinForms-type editing scenarios where you have an auto-generated file and a user-edited file contributing to the same type; I'm not sure those kind of situations apply in JavaScript/TypeScript.

[disshishkov]

Some classes can has many line numbers, just for better read splitting to several separate classes will help. You can split by different type, for example by logic (in case where this logic can not moved to another class), by visibility (private and public) and other. In case when combing of partial classes can has problems (for example 2 partial classes same the same method/variable declaration) compiler should notify and throw error.

[RyanCavanaugh]

This isn't really compelling. If your class is so large it can't be comfortably edited in a single file due to its sheer size, it's a very major design smell (e.g. comments in http://programmers.stackexchange.com/questions/157482). Splitting by different type/logic/visibility/etc is something that could be handled by an IDE or by organization within a single file.

It's worth discussing why your classes are so big they can't be navigated (is it because the navigation tools should be better?), and if there are better ways the language could support decomposing a class rather than just splitting it.

[rjamesnw]

Personally, I think "partial classes" should exist, but behave like modules that merge together. I have a system with modules that, although intellisense sees all modules (as it should), the actual JS containing it is only loaded when needed. I think that same would be great for classes as well - to only load the parts needed. I've also wanted to create a function, and use a class to further expand on it. Currently, you can only do this with modules.

[kvantetore]

A use case for partial classes is for generated proxy classes. E.g. WebAPI or SignalR wrappers. It would be really nice to be able to extend the generated proxy classes with custom logic. Especially when generating classes for models it would be nice to be able to attach business logic directly to the model classes returned from the api.

[wsmckenz]

+1 kvantetore.

The use case is exactly the same as .net; a portion of the class is generated (in our case, from Avro schemas) and you want to add additional helper code for working with the generated classes.

[yahiko00]

I like this suggestion. I would like to have partial classes in order to separate attributes/properties of classes forming my "data hierarchy" which could be gathered in one single file, from their methods which can be split in several other files. It would make code clearer and easier to understand in one glance imo.

My data hierarchy file:

class A {
  x: number;
  y: number;
  z: number;
}

class B extends A {
  value: string;
  flag1: boolean;
  flag2: boolean;
}

File containing methods of class A:

class A {
  constructor(x: number, y: number, z: number) {
    this.x = x;
    this.y = y;
    this.z = z;
  }
  method1(...) { ... }
  ...
  methodN(...) { ... }
}

File containing methods of class B:

class B extends A {
  constructor(x: number, y: number, z: number, value: string) {
    super(x, y, z);
    this.value = value;
    this.flag1 = false;
    this.flag2 = false;
  }
  method1(...) { ... }
  ...
  methodN(...) { ... }
}

[jfrank14]

+1 from me. I would like to use tt templates to generate Typescript classes, but add to them in a separate file that won't get replaced by the template generation.

[chrizy]

+1 This will really help me with my auto generated classes that I need to extend

[fletchsod-developer]

Partial Class is really nice but it doesn't make sense to have it. It showed you have a design problem. Rule of thumb in programming is to keep it simple stupid (KISS), regardless of languages you use, is to break apart large class into smaller ones - by either 1) splitting off scripts into classes and call them (Other classes can also call that smaller classes too instead of re-inventing the wheel), or 2) break apart & convert into Abstract/Interface/Inheritance/Virtual etc. or polymorphism.

Let's say you have a Vehicle with everything in it. A partial class doesn't make sense and it introduce complexity & overhead here where it makes more sense to do this instead. Create an Engine class, Tranny class, Drivetrain class, Door class & Tire class as seperate classes and move scripts over to them, that when called can still define a Vehicle within the Vehicle class. It reduce lengthy scripts & script complexity. Chance are you will find you have Door scripts here and there which can be simplified by a Door class. Interface/Abstract/Inheritance/Virtual can be use to alter the Door class definition on some scripts in the Vehicle class.

You're also more likely to have less develoment time this way. I once had the ASP.NET C# Blog that use lots of partial classes. I had struggled with it cuz of too many partial classes and you don't know where they are. It is sort of like dealing with GOTO logic in programming. Bad bad!! I was never able to create a patch for the bugfix successfully, nor was I able to customize the script cuz it was too buried in pile of partial classes (so do some renamed wording).

Just saying it as a 1 cent thought from my mind.

[jfrank14]

@fletchsod-developer: Some situations are properly handled by inheritance, but not all. And some developers may misuse partial classes, but not all. There are some situations where I find partial classes very, very useful, but if you don't like them, you don't have to use them.