Class decorators and async process

Brendan Eich brendan at
Sat May 23 18:54:35 UTC 2015

Gray Zhang wrote:
> Hi all:
> I’m wondering if there is any way to combine a class decorator and an 
> async process together? A common case would be a generic IoC 
> implementation:

First, too many undefined terms and made-up syntax extensions just makes 
for confusion. Can you define "async process"? To optimize I'll assume 
you mean async function, a function returning a promise.

> Since in JavaScript IoC

IoC = Inversion of Control -- just checking!

> we load runtime modules async by a config file, the interface may be:
> |{Promise} ioc.getComponent({string} componentName)
> |

Nicer to use reserved type annotation syntax:

|  ioc.getComponent(componentName: string): Promise|

I hope that's what {T} D means!

> which resolves the returned Promise giving required instance of 
> |componentName|, and this is a method that cannot be sync since we 
> need a config file to map |componentName| to its implementing module 
> and load the module lazily (for performance reason)
> This is an async process so if we add a decorator to a class property:
> |function inject(name) {
>      return (target, key, descriptor) {|

Missing `function` after `return`?

> |
>          // Note this returns a promise, not the actual property value
>          descriptor.initializer = () =>  ioc.getComponent(name);
>      }
> }
> class Hero {
>      @inject('knife')
>      weapon = null|

So the decorator calls the anonymous function with key='knife', 
target=instance-of-Hero, and descriptor the property descriptor, 
extended with .initializer?

BTW the idea of using property descriptors for decorators got push-back 
and an alternative suggestion at the March TC39 meeting.

> |
>      hit(enemy) {
>          enemy.heath -= (this.weapon.power - enemy.defense);
>      }
> }
> |
> This code may not work, but nobody likes there injected properties to 
> be all async getters and all code logics become unnecessarily complex 
> by introducing so many async processes

Have you read about `await` and `async` in ES7?

> How so we think of such common case, should I just make the 
> |ioc.getComponent| sync without considerations to performance,

"performance" is not really accurate: responsiveness and even deadlock 
avoidance come to mind. If you're talking about a browser API, then you 
can't do sync loading (apart from bad old sync XHR), so async/await is 
the way to go.

> or should I give up the decorator solution?

Is the decorator idea based on extended property descriptors actually 
being used, e.g., with Babel? Sorry if I missed it.


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