I'm a new developper in Node.js (ing 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 (), 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 !

I'm a new developper in Node.js (ing 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./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 !

• javascript
• node.js
• ecmascript-6
• decorator

• 3 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 Commented Dec 7, 2019 at 18:13
• 1 I e from java and I was looking decorators for dependency injection. As @sami said, what is your requirement? Also you can check this github./jrichardsz/dependency-injection-4nodejs – JRichardsz Commented Dec 7, 2019 at 18:17
• @JRichardsz what's the benefit of your "injection" over module.exports = express => { express.get(/*...*/) /*...*/ } ? – Jonas Wilms Commented Dec 7, 2019 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 Commented Dec 7, 2019 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 stackoverflow./a/3153617/3957754 and this raw.githubusercontent./jrichardsz/static_resources/master/… – JRichardsz Commented Dec 7, 2019 at 19:29

2 Answers 2

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);
  };

@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.