How to create an instance of a class if I'm passing its module into the constructor instead of requiring it?

Question

To make instances of one ES6 class available in another, I've often used this structure:

const Something=require('./something');

class AnotherClass{

  makeASomething(){
      var outputSomething=new Something();
      return outputSomething;
  }

}

module.exports=AnotherClass;

However, I have a class where instead of importing the module in a require() above the class definition, I'm passing it into the constructor, then in the same class I'm creating an instance of that class for use in a REST endpoint:

class AnotherClass{

  constructor(Something){
      this.Something=Something;
  }


  initialize(app){
    app.post('/sendSomething',async function(req,res){
        const response=new this.Something();
        res.end(response.toString());
    });
  }



  makeASomething(){
      var outputSomething=new this.Something();
      return outputSomething;
  }

}

module.exports=AnotherClass;

Which I want to do so that I can do dependency injection and pass in a version of Something with mock methods.

But the latter version is giving me this error:

TypeError: Cannot read property 'Something' of undefined

So I guess there's something wrong with how I am trying to pass the module into the constructor. How do I pass it in so that I can create instances of Something in the methods of AnotherClass?

EDIT: added code to show how I'm actually creating the instance of Something.

Can you provide a condesanbox? What you are doing works for me. — Johnny Zabala, Feb 04 '20 at 11:39
The error indicates that `this` doesn't refer to the class instance, so you are not calling `makeASomething` correctly. Please show us how you call it. Possibly a duplicate of [How to access the correct `this` inside a callback?](https://stackoverflow.com/q/20279484/218196) — Felix Kling, Feb 04 '20 at 12:35
@FelixKling, added code showing that I'm calling the `new this.Something()` constructor in an `app.post()` endpoint. — sigil, Feb 05 '20 at 09:44
You're using `function` which changes context. Please use `async (req, res) => {...}` in your `app.post()` endpoint — Sebastian Kaczmarek, Feb 05 '20 at 09:47

score 2 · Accepted Answer · answered Feb 05 '20 at 10:12

2

It's because you have used function in the app.post() endpoint. You need to use an arrow function in order to make this refer to the AnotherClass instance:

  initialize(app){
    app.post('/sendSomething', async (req, res) => {
        const response = new this.Something();
        res.end(response.toString());
    });
  }

answered Feb 05 '20 at 10:12

Sebastian Kaczmarek

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2

This is the right way. Forgot that this is one of the benefits of using arrow functions. – RyanDay Feb 05 '20 at 10:16

score 1 · Answer 2 · answered Feb 05 '20 at 10:09

You've got a scope problem. this in the post function is not referring to AnotherClass. You can get around this by storing the value in a variable before calling your post function:

class AnotherClass{

constructor(Something){
    this.Something=Something;
}


initialize(app){

    const _something = this.Something;

    app.post('/sendSomething',async function(req,res){

        const response=new _something();

        res.end(response.toString());
    });
}



makeASomething(){
    var outputSomething=new this.Something();
    return outputSomething;
}

}

module.exports=AnotherClass;

How to create an instance of a class if I'm passing its module into the constructor instead of requiring it?

2 Answers2