What is the best way to generate a comprehensive list of values associated with a specific key within an array of objects?

Let's consider a scenario where we have the following interface:

interface OrderPurchase {
  headerName: string;
  minWidth: number;
}

We also have an array of objects as shown below:

const columns: OrderPurchase[] = [
  {
    headerName: 'ID',   
    minWidth: 78,
  },
  {
    headerName: 'Email',
    minWidth: 250,
  },
  {
    headerName: 'Name',
    minWidth: 180,
  },
  {
    headerName: 'Country',
    minWidth: 80,
  },
  {
    headerName: 'Total',
    minWidth: 80,
  }];

The question arises on how to define a type HeaderName that is restricted to specific values used in the key within the array. In this particular case, HeaderName should only be allowed to take on values such as ID, Email, Name, Country, or Total.

An attempted solution was made using the code snippet below, but it seems to only limit values to string:

type HeaderName = typeof columns[number]['headerName'];

Answer №1

The issue lies in the fact that the columns variable is not explicitly typed, requiring us to remove its type declaration. However, even after removing the type declaration, the compiler widens the types to their primitive values. If we were to check the type of the columns variable, it would be:

type HeaderName = {
    headerName: string;
    minWidth: number;
}[]

To prevent this widening behavior, we can utilize const assertion, and for ensuring type safety, we can use the satisfies operator.

const columns = [
  {
    headerName: 'ID',
    minWidth: 78,
  },
  {
    headerName: 'Email',
    minWidth: 250,
  },
  {
    headerName: 'Name',
    minWidth: 180,
  },
  {
    headerName: 'Country',
    minWidth: 80,
  },
  {
    headerName: 'Total',
    minWidth: 80,
  },
] as const satisfies readonly OrderPurchase[];

It's important to note that including the keyword readonly is essential, as the const assertion makes all properties read-only.

Here is a test case:

//  "ID" | "Email" | "Name" | "Country" | "Total"
type HeaderName = typeof columns[number]['headerName'];

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