TypeScript destructure array of objects, both first object and a value within it

Question

I have an array of objects I want to destructure, retrieving both the first object and a value within it:

const [{ a }] = ([firstObjectInArray] = [
  {
    a: 1,
  },
  {
    b: 2,
  },
]);
console.log(a); // 1
console.log(firstObjectInArray); // { a: 1 }

In Javascript this works; but in TypeScript this returns

Cannot find name 'firstObjectInArray'.ts(2304)

I'm trying to figure out how to type that in order to avoid the error.

Why not do it 2 separate destructuring as mentioned in your [previous question](https://stackoverflow.com/q/65354700/): `const [first] = [{ a: 1 }], { a } = first`. — adiga, Dec 18 '20 at 10:12
In the current question, what is the issue with using `const [first] = someArray, { a } = first`? — adiga, Dec 18 '20 at 10:15
The example provided in this question works in Javascript; I'm wondering where the TypeScript error comes from. — Emille C., Dec 18 '20 at 10:19

score 1 · Accepted Answer · edited Dec 18 '20 at 10:20

1

As firstObjectInArray isn't part of your declaration (it's just an expression), it's an assignment to an undeclared variable.

To solve the issue, you have two ways:

Do it in two steps:

const [firstObjectInArray] = [
    {
      a: 1,
    },
    {
      b: 2,
    },
  ];
const {a} = firstObjectInArray
console.log(a); // 1
console.log(firstObjectInArray); // { a: 1 }

Declare the firstObjectInArray earlier:

let firstObjectInArray; //<-- This can't be made `const`, as it has no initializer
const [{ a }] = ([firstObjectInArray] = [
  {
    a: 1,
  },
  {
    b: 2,
  },
]);
console.log(a); // 1
console.log(firstObjectInArray); // { a: 1 }

edited Dec 18 '20 at 10:20

Bergi

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answered Dec 18 '20 at 10:18

FZs

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Dont like any of those solutions, but explains the issue. Thanks. – Emille C. Dec 18 '20 at 10:20
1

The first one *is* the proper solution (and superior to the second). You can even do it in one declaration: `const [firstObjectInArray] = …, {a} = firstObjectInArray;`. – Bergi Dec 18 '20 at 10:21
@Bergi can you elaborate? – Emille C. Dec 18 '20 at 10:23
@Bergi I think you are wrong, that's not possible. – Emille C. Dec 18 '20 at 10:31
@EmilleC. What do you mean by "not possible"? – Bergi Dec 18 '20 at 10:32
@EmilleC. Any variable declaration (`var`, `let` and `const`) can define any number of variables separated by commas... – FZs Dec 18 '20 at 10:37
Yes, that was proposed in above; but the question is not about this. – Emille C. Dec 18 '20 at 10:40
@EmilleC. I know, just stated that it **is** possible (the thing what Bergi said). – FZs Dec 18 '20 at 10:41

TypeScript destructure array of objects, both first object and a value within it

1 Answers1