Type 'string[]' cannot be used as an index type.ts(2538)

Question

TS Error:

Type '(string | undefined)[]' cannot be used as an index type.ts(2538)

Code:

Object.values(payload.reduce((acc, obj) => {
  let key = obj.values.map(i => i.object_id);
  if (!acc[key]) {
    acc[key] = []
  }  
  acc[key].push(obj)
  return acc
}, {}))

Works fine in the javascript code. Why?

This answer should explains things for you: stackoverflow.com/questions/43836274/… — Koronag
– Koronag, Commented Jan 19, 2020 at 1:44
I'm not familiar with TypeScript, but JavaScript implicitly coerces the array in key into a string when key is used as an object property. TypeScript probably doesn't like this for whatever reason. You can likely resolve this problem by explicitly doing key = key.toString(); to stop TypeScript from complaining. — B. Fleming
– B. Fleming, Commented Jan 19, 2020 at 1:58
@Koronag, I tried use non null assertion operator (i => i.object_id!). This led to an error "Type 'string[]' cannot be used as an index type.ts(2538)" — Andrey Luzin
– Andrey Luzin, Commented Jan 19, 2020 at 14:10
@B.Fleming Typescript doesn't like it for the reason that doing so is literally the reason Typescript exists — millimoose
– millimoose, Commented Jan 20, 2020 at 3:29

KingDarBoja · Accepted Answer · 2020-01-20 03:38:13Z

1

Looks like you are trying to access an array value by using an array key as this line:

let key = obj.values.map(i => i.object_id);

Returns an array of strings and assign it to the key variable.

EDIT What I mean is that you can't use an array as index. Try to assign a string value to key instead of the mapping result.

Something like this:

Object.values(list.reduce((acc,obj) => {
  let keys = obj["values"].map(i => String(i.object_id));
  console.log(`Result mapping key`);
  console.log(keys);

  for (const subkey in keys) {
    if (!acc[subkey]) {
      acc[subkey] = []
    }
    acc[subkey].push(obj);
  }
  return acc
}, {}));

edited Jan 20, 2020 at 3:38

answered Jan 19, 2020 at 3:53

KingDarBoja

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2 Comments

Andrey Luzin Over a year ago

key return array of strings. Example: codepen.io/Luzin/pen/bGNQGGB

KingDarBoja Over a year ago

Sorry, maybe I wasn't clear about the issue, I will edit my answer.

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