How to access class parameter in anonymous object in Typescript?

Question

How can I access class members during the creation of an anonymous object?

class foo {
   private working = 'hello';
   private notWorking = 'world';

   public method() {
      new DoSomethingClass(working, {
         ...,
         this.notWorking //this throws an error as this has a different scope now
      });
   }
}

Did you try the approach to store this in another variable before the scope change? E.g. const self = this; right before you do new DoSomethingClass and then use self within the new scope. It may still not be possible to access a private property that way though. — k0pernikus
– k0pernikus, Commented Sep 9, 2019 at 12:19
Your class syntax is also wrong. The : are not valid there. — k0pernikus
– k0pernikus, Commented Sep 9, 2019 at 12:21
And it also may help to describe what you are trying to achieve. What do you want to inject in the DoSomethingClass? I am confused as to why you want to add the logic inside the foo class and not the DoSomethingClass. — k0pernikus
– k0pernikus, Commented Sep 9, 2019 at 12:23
I fixed the ":", this was just a typo. TheDoSomethingClass is from a lib and takes a parameter object. I want to pass the arguments to it that i have stored in a class. — iuiz
– iuiz, Commented Sep 9, 2019 at 12:24
I think you are missing the key of your config object. {theKeyTheLibWants: this.notWorking} — k0pernikus
– k0pernikus, Commented Sep 9, 2019 at 12:28

k0pernikus · Accepted Answer · 2019-09-09 12:59:44Z

4

When passing a config object, make sure it uses the right key:

  new DoSomethingClass(working, {
     theRequiredKey: this.notWorking
  });

Check your library for the key names.

You can still use a shortcut for that you use the same variable name as the key is called, e.g.:

  const theRequiredKey = this.notWorking;
  new DoSomethingClass(working, {
     theRequiredKey, // will create an object with key "theRequiredKey" and its value
  });

This will work as wanted.

Also check if your library has typescript typings available, e.g. via

npm install @types/{library-name}

or create your own typings.

edited Sep 9, 2019 at 12:59

answered Sep 9, 2019 at 12:44

k0pernikus

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Comments

Matthias · Accepted Answer · 2022-03-21 18:29:12Z

At runtime there is no way to guarantee that this will have the right scope. A function's scope (proper term is "context") can be customized at call time with bind/call/apply, and if separated from it's object, it loses its original context:

var fn = instance.method;
fn(); // `this` will not be `instance` anymore

Arrow functions allow you to avoid this issue.

public method = () => { ... }

There are other ways like stashing var _this = this, but that's error prone.

Note: arrow class methods are different than regular class methods. Since they need to access instance variables, the typescript-generated function will not be on the prototype, but rather in the class constructor. Each instance will have a separate function instance, instead of all pointing to the same function instance. This might affect performance in hot code paths.

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