Using decorators in Node.js

Question

I'm a new developper in Node.js (coming from Python), and I'm quite surprised to see that there are no decorators in Javascript. I would like to use it, however, because it greatly simplifies the code.

After some research, I found an ES6 specification in stage 2 for this (https://github.com/tc39/proposal-decorators), but it is apparently not supported in Node.js. I also found this in TypeScript, but their function is limited (only in classes).

So my question is: Is there a way to have decorators similar to those of Python with Node.js, and if not, what features can be used as a substitute ?

Thank you in advance for your answers !

Might be better to explain what you want to do that you think is simpler with decorators. Someone might be able to explain how to do it in JavaScript — Sami Kuhmonen, Dec 07 '19 at 18:13
I come from java and I was looking decorators for dependency injection. As @sami said, what is your requirement? Also you can check this https://github.com/jrichardsz/dependency-injection-4nodejs — JRichardsz, Dec 07 '19 at 18:17
@JRichardsz what's the benefit of your "injection" over `module.exports = express => { express.get(/*...*/) /*...*/ }` ? — Jonas Wilms, Dec 07 '19 at 18:38
I need decorators to check args of an function before its execution. I think their syntax is very great for doing that. @SamiKuhmonen — baptiste0928, Dec 07 '19 at 18:50
#JonasWilms benefit is that you don't need to pass express explicitly to another javascript modules, you just need to use it because the **dependency injection core** must instantiate it at the beginning of your app. If you can, check this https://stackoverflow.com/a/3153617/3957754 and this https://raw.githubusercontent.com/jrichardsz/static_resources/master/dependency-injection/dependency-injection.png — JRichardsz, Dec 07 '19 at 19:29
Just for your information: https://node.green/ ... great side for learning what's available in Node.js ... for decorators, nothing's available yet: https://node.green/#ESNEXT-Stage-2-Class-and-Property-Decorators — Thomas Urban, Mar 16 '23 at 20:32

Jonas Wilms · Accepted Answer · 2019-12-07T18:42:28.733

4

According to Wikipedia decorators in python are just syntactic sugar for a function call, which takes the original class/function and stores the returned value under the variable containing the class. Thus the equivalent js would be:

  const Decorated = decorator(class ToBeDecorated {
     /*...*/
  });

  const decoratedFunction = decorateFunction(function decorated() {
     /*...*/
  });

  // Some sample decorator implementations:
  const decorator = Parent => class WithDuck extends Parent {
   quack() { }
  };

  const decorateFunction = fn => (...args) => {
     console.log(`${fn.name} called with`, ...args);
     return fn(...args);
  };

edited Dec 07 '19 at 18:42

answered Dec 07 '19 at 18:29

Jonas Wilms

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Is it possible to directly call a function decorated with this system ? – baptiste0928 Dec 07 '19 at 18:53
@baptiste0928 `decoratedFunction()` ? – Jonas Wilms Dec 07 '19 at 18:57

score 0 · Answer 2 · answered Sep 12 '22 at 21:24

@jonas Wilms 's answer is correct. I wanted to post this here to expand on a specific implementation technique I used. My use case was with Koa, using async/await functions.

First, I created my decorator:

export function Cocoon( { param=[] }, callback )
{
    //Return a function from the decorator
    return async (ctx) => {
        //Do a thing with Param
        const params = { usr: param }

        return await callback( ctx, params )
    }
}

Using the decorator to annotate a function:

export const list = Cocoon({ 
        param: [5]
    },
    async (ctx, { usr }) => {
        //Function body
        ctx.response.body = usr
    }
)

Now I can export 'list' to Koa and it'll be called as expected.

Using decorators in Node.js

2 Answers2