Appending a string to a TypeScript object type, where property names are derived from a string literal type

Question

I would like to dynamically define a type like this from a list of currency ids:

{
  usd: number;
  usd_mcap: number;
  eur: number;
  eur_mcap: number;
  gbp: number;
  gbp_mcap: number;
}

I have the following, but I cannot figure out how to add the {currency}_mcap property to the object:

type CurrencyId = 'usd' | 'eur' | 'gbp';
type AssetInfo = {
  [key in CurrencyId]: number;
} & {
  [(key + '_mcap') in CurrencyId]: number; // This does not work
}

Any ideas what I can try?

davecardwell · Accepted Answer · 2022-07-25 18:03:03Z

1

You can use Template Literal Types for this:

type CurrencyId = 'usd' | 'eur' | 'gbp';

// as a Record
type AssetInfoRecord = Record<CurrencyId | `${CurrencyId}_mcap`, number>;

// as an Object
type AssetInfoObject = {
  [key in CurrencyId | `${CurrencyId}_mcap`]: number;
};

answered Jul 25, 2022 at 18:03

davecardwell

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1 Comment

romiem Over a year ago

Thank you, that has worked perfectly. I had no idea that Template Literal Types even existed!

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