Typescript: destructuring an object with symbols as keys

Question

Why this code produces an error Type 'symbol' cannot be used to index type '{ [x: string]: string; }'.:

let symbol = Symbol()
let obj = { [symbol] : 'value'}
let { [symbol]: alias } = obj
             // ^^^^^ the error is here

console.log(alias)

And most importantly, how do I fix this?

@zahma I don't think too. But that's not an array, but a syntax for computed props and symbols. — Nurbol Alpysbayev, Jan 02 '19 at 10:22

score 2 · Accepted Answer · answered Jan 02 '19 at 10:20

2

You just need to declare the symbol as const to make the compiler infer a literal type for it and not the general Symbol type.

const symbol = Symbol()
let obj = { [symbol] : 'value'}
let { [symbol]: alias } = obj


console.log(alias)

This PR might be useful as to when typescript infers a unique symbol

answered Jan 02 '19 at 10:20

Ha! So simple. And elegant, as always. There is a very important thing you said but I am not sure if I've understood it correctly. Are you saying, that `let` and `const` produce different types? Is this (Symbols) the only case? – Nurbol Alpysbayev Jan 02 '19 at 10:25
Also are *a literal type* and *unique symbol* the same concepts in this context? – Nurbol Alpysbayev Jan 02 '19 at 10:28
2

@NurbolAlpysbayev generally `let x = litreral` will infer the widened type while `const x = literal` will infer the literal type. So `let x = 'foo'` x is `string` while `const x = 'foo'` x is typed as `'foo'` – Titian Cernicova-Dragomir Jan 02 '19 at 10:29
Makes sense! Funny enough, I almost always use `const`, but today for some reason wrote `let` and here we are. – Nurbol Alpysbayev Jan 02 '19 at 10:31
@NurbolAlpysbayev you change `const` to `let`, ask an interesting question, we all get point, it all works out :P – Titian Cernicova-Dragomir Jan 02 '19 at 10:51
Answers usually get more upvotes in the long run, who knows. If you feel like chatting check your gitter :) – Titian Cernicova-Dragomir Jan 02 '19 at 10:54
Eh, I don't know what you mean, I don't have an account there ) We'd just erase our comments, and that's it! If you meant to not pollute SO with them ) – Nurbol Alpysbayev Jan 02 '19 at 10:56
@NurbolAlpysbayev if you have a GitHub account you kinda have a https://gitter.im/ one too :) – Titian Cernicova-Dragomir Jan 02 '19 at 10:58

1 Answers1