Question 168854 in Javascript Question posted in
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What are the keys of an object in Javascript?

I have the following object:

type Currencies = "tl" | "us-dollar" | "euro"
const currenciesMap = {}

I want to say that the keys of this object must be one of the values defined in the Currencies type.

So the object is going to seem as follows:

const currenciesMap = {
  tl: "₺",
  "us-dollar": "$",
  euro: "€"
}

So typescript should only allow these keys to be defined.

How to make this type?

I thought this would work but it didn’t:

const currenciesMap: { [key: Currencies[string]]: string } = {}

Typescript shows the following error:

An index signature parameter type must be 'string', 'number', 'symbol', or a template literal type.
Tags:

3

Answers


  1. 0

    You don’t need to access Currencies with [string]. You should just say K in Currencies, as follows:

    type Currencies = 'tl' | 'us-dollar' | 'euro';

const currenciesMap: { [K in Currencies]: string } = {
  tl: '₺',
  'us-dollar': '$',
  euro: '€',
};
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  2. 0

    You are probably looking for Typescript Record utility type. It is documented here

    Record is equivalent to

    type Record<K extends string | number | symbol, T> = { [P in K]: T }

    type Currencies = "tl" | "us-dollar" | "euro"
const currenciesMap: Record<Currencies, string> = {
  tl: "₺",
  "us-dollar": "$",
  euro: "€"
}

    Note that when declaring your variable as a record this way, you will be forced to declare all the properties possible. If you don’t want to have them all, you could wrap it around Partial

    type Currencies = "tl" | "us-dollar" | "euro"
const currenciesMap: Partial<Record<Currencies, string>> = {
  tl: "₺",
  "us-dollar": "$",
}
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  3. 0 
    
const currencies = {
  tl: "₺",
  "us-dollar": "$",
  euro: "€"
} as const


type Currency = typeof currencies[keyof typeof currencies]

    Usage

    const log = (currency: Currency) => console.log(currency);
log(currencies.euro)

    Or:

    log("€")

    More info:

    What does the "as const" mean in TypeScript and what is its use case?

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