TypeScript how to convert an Array of objects to a concrete model?

Question

I have this simple example below where I want to convert an array of objects to an array of Animal objects but I get this error Type 'Animal[]' is missing the following properties from type 'Promise<Animal[]>': then, catch, [Symbol.toStringTag] in the Controller.getPersons() function. I am not entirely sure what is causing this error.

class Animal {
  name: string;
  colour: string;

  constructor(name: string, colour: string) {
    this.name = name;
    this.colour = colour;
  }
}

The class where I have this function which promise to return an array of Animal objects getPersons(): Promise<Animal[]>

class Controller {
  data: { name: string; colour: string }[];

  constructor(data: { name: string; colour: string }[]) {
    this.data = data;
  }

  getPersons(): Promise<Animal[]> {
    const animals = this.data.map(a => new Animal(a.name, a.colour));
    console.log("animals -----> ", animals);
    console.log("type -----> ", typeof animals);
    return animals;
  }

This is my sample data which I want to convert to an array of Animal objects

const data = [
   { name: "Dog", colour: "Black" },
   { name: "Cat", colour: "White" }
];

const c = new Controller(data);
c.getPersons();

I would appreciate any help. Thank you in advance.

I believe your issue is you are returning Animal[] as opposed to Promise<Animal[]>. Either alter the return type of Promise<Animal[]> to be Animal[] or update data to return a promise. — ttugates
– ttugates, Commented Sep 7, 2019 at 16:09

lukasgeiter · Accepted Answer · 2019-09-07 16:35:12Z

1

Your method getPersons() has a return type of Promise<Animal[]>. However what you're actually returning is just an array of animals.

As @ttugates points out in the comments, you have two options:

Change the return type

Change the return type of the method to Animal[] to match the implementation:

getPersons(): Animal[] {
    // ...
}

Change the implementation

If you really need a promise, possibly to conform to a certain interface, create one and return it:

getPersons(): Promise<Animal[]> {
    const animals = this.data.map(a => new Animal(a.name, a.colour));
    console.log("animals -----> ", animals);
    console.log("type -----> ", typeof animals);
    return Promise.resolve(animals);
}

answered Sep 7, 2019 at 16:35

lukasgeiter

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TypeScript how to convert an Array of objects to a concrete model?

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Change the return type

Change the implementation

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