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[Request] Support object composition via structs
microsoft
TypeScript, Dockerfile
Composition over inheritance is something that has been a part of JavaScript since the dawn of time. While it’s debatable whether it is or isn’t an effective way to approach a problem, it’s still a part of the daily lives of many JavaScript developers.
This ticket isn’t here to discuss the use-cases appropriate for composition over inheritance, instead this ticket is here to plead for the introduction of a mechanism to use it within the TypeScript we know and love.
A simplified form in normal JSwould look like:
```typescript
const createA = () => ({
actionA() {
console.log(‘Hi from A’)
}
})
const createB = () => ({
actionB() {
console.log(‘Hi from B’)
}
})
const createComposite = () => ({
...createA(),
...createB(),
})
const composite = createComposite()
composite.actionA()
composite.actionB()
```
This works fine in Typescript too, however dealing with composition through classes is super cumbersome as it involves redeclaring types and proxying behaviours and properties.
```typescript
class A {
actionA() {
console.log(‘Hi from A’)
}
}
class B {
actionB() {
console.log(‘Hi from B’)
}
}
class Composite {
private a: A
private b: B
constructor(){
this.a = new A()
this.b = new B()
}
actionA() {
this.a.actionA()
}
actionB() {
this.b.actionB()
}
}
```
As you can imagine, this redeclaration becomes annoying to deal with as requirements grow.
The TypeScript community has come up with solutions such as mixins or decorators, but there is no simple and smooth solution.
While I know it's important to the TypeScript team to not take features from other languages, currently there is a hole in TypeScripts support for compositional approaches and thus I feel it's important we look to people who have seen this challenge and addressed it in a way which feels JavaScript-y and preserves the consistency of TS.
Thankfully, Go/Golang is surprisingly JavaScripty and the developers behind the project have considered and created a solution to this. Their solution was to declare a struct object type which can be given methods and composed with other struct objects.
The great thing is that the implementation is essentially 1:1 syntax sugar for factory functions.
Dig it:
interactive example:
https://play.golang.org/p/sskWaTpJgr
```go
type A struct {}
func (A) actionA() {
fmt.Println(“Hi from A”)
}
type B struct {}
func (B) actionB() {
fmt.Println(“Hi from A”)
}
type Composite struct {
A,
B,
}
```
To break it down a little
```go
// Declare an object
type A struct {}
/*
Assign a function to that object
func (targetObject) name (...params) {
...body
}
*/
func (A) actionA() {
fmt.Println(“Hi from A”)
}
```
I'm not going to suggest syntax but I do want to illustrate that the typescript implementation can be more JavaScript-y
```typescript
struct A {
actionA() {
console.log(‘Hi from A’)
}
}
struct Composite {
...A
}
const createComposite = () => Composite{}
const composite = createComposite()
```
## Checklist
My suggestion meets these guidelines:
* [x] This wouldn't be a breaking change in existing TypeScript/JavaScript code
* [x] This wouldn't change the runtime behavior of existing JavaScript code
* [x] This could be implemented without emitting different JS based on the types of the expressions
* [x] This isn't a runtime feature (e.g. library functionality, non-ECMAScript syntax with JavaScript output, etc.)
* [?] This feature would agree with the rest of [TypeScript's Design Goals](https://github.com/Microsoft/TypeScript/wiki/TypeScript-Design-Goals).
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