Typescript - clone an object and cast into another type

Question

I am trying to clone an untyped object. I want to cast it while cloning. Here is a very simple code:

const typedOject: type123 = {...untypedObject} as type123;
typeObject.a = 1;

export interface type123 {
   a: number;
   b: number;
}

But the compiler complains: Property 'a' does not exist on type 'any[]'

So, why the new object is of type any?

Thanks

For tasks like this, I typically write a converter function that takes an any object and checks its attributes + builds up the return type — Derek Pollard
– Derek Pollard, Commented Oct 21, 2020 at 20:02
I see. I've added an answer, please let me know if you have any other questions — Derek Pollard
– Derek Pollard, Commented Oct 21, 2020 at 20:14

Derek Pollard · Accepted Answer · 2020-10-21 20:13:20Z

These types of conversions are common for handling API data coming in

Here's how I handle these type of conversions:

types/GithubRepo.ts

export interface GithubRepo {
  id: number;
  name: string;
  description: string;
  stars: number;
}

export const convertIntoGithubRepo = (githubRepoFragment: any): GithubRepo => ({
  id: githubRepoFragment.id,
  name: githubRepoFragment.name,
  description: githubRepoFragment.description || '',
  stars: githubRepoFragment.stargazers_count,
});

Usage


import { convertIntoGithubRepo } from 'types/GithubRepo';


const githubRepo = convertIntoGithubRepo(someGithubRepoFragment);

For this example, I am defining a type, then defining methods to copy an object of type any over to a type of GithubRepo

During this conversion, we can see that I'm checking for a descriptions existence on the any object, and adding a blank if one is falsy

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Typescript - clone an object and cast into another type

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