Typescript error message TS2693: The identifier 'Note' is used as a value although it refers to a type

I have been attempting to incorporate Typescript into my Firebase project, but unfortunately I am encountering the following errors:

 error TS2693: 'Note' only refers to a type, but is being used as a value here.

The code snippet I used is as follows:

interface Note {
 image: string;
 name: string;
 pdf: string;
}

const itemname = Note.name;

Unfortunately, this approach does not seem to be effective.

Below are the import statements:

import {
AngularFirestore,
AngularFirestoreCollection,
AngularFirestoreDocument,
} from '@angular/fire/firestore';
import { Component, OnInit } from '@angular/core';
import { Observable } from 'rxjs';
import 'rxjs/add/operator/map';

Here is another segment of the code:

  export class Advance11Component implements OnInit {
  notesCollection: AngularFirestoreCollection<Note>;
  notes: Observable<Note[]>;

  constructor(private afs: AngularFirestore) {}
  ngOnInit(): void {
  this.notesCollection = this.afs
    .collection('class11AdvancedTheory')
    .doc(itemname)
    .collection(itemname, (ref) => {
      return ref;
    });
    this.notes = this.notesCollection.valueChanges();
   }
  }

Answer №1

The structure of something can be defined by an interface (or type).

// A `car` has a certain number of `wheels`.
interface Car {
  wheels: number
}

An example (e.g. const vehicle = ...) determines a specific value that can be accessed during runtime.

// This `car` has 4 `wheels`.
const myCar: Car = { wheels: 4 }

You should not confuse these two concepts when assigning values. For instance, if you were to question "How many wheels does myCarWheels have?” the response would be “typeof Car.wheels is number”, which is irrelevant in JavaScript.

// Incorrect usage
const myCarWheels = Car.wheels // typeof `Car.wheels` is `number`.

You may be interested in one of these alternatives:

interface Car {
    wheels: number
}

type CarWheels = Car['wheels'] // extract a property from an interface

const myCar: Car = { wheels: 4 } // create an instance of `Car`

const myCarWheels: Car['wheels'] = 4 // define a variable that could be assigned to 
                                      //    an instance of `Car`

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