TypeScript does not recognize the type of a deconstructed object

When retrieving a collection from Firebase, I use the following basic approach:

import { getFirestore, collection, getDocs } from "firebase/firestore";

import { Invoice } from "../models/Invoice";

const db = getFirestore();

export const getInvoicesCollection = async () => {
  try {
    const snapshot = await getDocs(collection(db, "invoices"));

    const docs: Invoice[] = snapshot.docs.map((doc) => ({
      ...doc.data(),
    }));

    console.log(docs);
  } catch (error) {
    console.error(error);
  }
};

export default getInvoicesCollection;

To refine the response object received from Firebase, specifically mapping only what is needed into a new object, this portion of code executes:

const docs: Invoice[] = snapshot.docs.map((doc) => ({
  ...doc.data(),
}));

console.log(docs);

The outcome is displayed in the browser as shown here:

https://i.sstatic.net/qiwbe.png

However, TypeScript raises an error message stating:

Type '{ [x: string]: any; }[]' is not assignable to type 'Invoice[]'.
Type '{ [x: string]: any; }' is missing the following properties from type 'Invoice': id, createdAt, paymentDue, description, and 8 more.

To address this issue, it seems that the variable is interpreted as an array of objects with a single property being a string capable of holding any value. How can I provide proper definition for the object structure?

Answer №1

When doc.data() is used, it returns a type of { [x: string]: any; }. This means that the result is an object with properties that have string keys and values of any type. Due to this flexibility, TypeScript cannot determine a more specific type as the database could potentially return any kind of data.

Therefore, in order to specify the type, you will need to assert it explicitly:

const docs: Invoice[] = snapshot.docs.map((doc) => ({
  ...doc.data() as Invoice,
}));

It's important to note that by using a type assertion, you are essentially bypassing TypeScript's type checking and assuming responsibility for ensuring the correctness of the types being used. If there is a mistake and the database contains different types of objects than expected, TypeScript will not be able to detect it.

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