Index signature for type 'string' is missing

February 6, 2022

I am trying to type my data in Firebase Cloud Functions using Typescript:

type SpotPriceByDay = {
  NOK_per_kWh: number;
  valid_from: string;
  valid_to: string;
}
type SpotPrices = {
  [date: string]: SpotPriceByDay
}

My Typescript compiler gives me an error when trying to use this

let data: SpotPrices = response.data;
data = Object.keys(data).map((key) => {
  const newKey = new Date(key).toISOString();
  return {[newKey]: data[key]};
});

error TS2322: Type '{ [x: string]: SpotPriceByDay; }[]' is not assignable to type 'SpotPrices'.
Index signature for type 'string' is missing in type '{ [x: string]: SpotPriceByDay; }[]'.

I have tried to understand how I can fix this error, but I have no clue. Any help would be appreciated!

>Solution :

data is a map of type SpotPrices but the map function here returns an array of SpotPrices objects. Assigning the result of the map should be a way to resolve this:

const data: SpotPrices = response.data;

const parsedData = Object.keys(data).map((key) => {
  const newKey = new Date(key).toISOString();
  return { [newKey]: data[key] };
});

Another way would be changing type of data to either SpotPrices or SpotPrices[]:

let data: SpotPrices | SpotPrices[] = response.data;

data = Object.keys(data).map((key) => {
  const newKey = new Date(key).toISOString();
  return { [newKey]: (data as SpotPrices)[key] };
});