How to make an object type / interface based on an array of strings

Question

I am getting some JSON objects from a network call, of the shape:

{
  timestamp: "1636814680595",
  price: "$1",
  ...manyOtherKeys
}

All values are strings.

I'd like to create an interface based on an array of the expected keys. From this:

const expectedKeys = ['timestamp', 'price', ...]

I would like to programmatically generate the type (or interface):

type ExpectedType {
  timestamp: string, 
  price: string
}

Is there an easy way to do that? For now, all I can think of is the following, which seems contrived:

// I did not find a way to start from an array and reduce it to this object
// while preserving the type. I'm okay starting with this object instead of
// an array of string. 
const objectWithExpectedShape = {
  timestamp: '',
  price: '',
}
type ExpectedType = typeof objectWithExpectedShape;

I don't have an answer for you, but something that might help is the io-ts library. github.com/gcanti/io-ts. I currently use this to validate data coming from the client (from JSON, like your example) to coerce them into real domain models that conform to a strict interface. This could help you. — luxdvie
– luxdvie, Commented Nov 13, 2021 at 15:00

CertainPerformance · Accepted Answer · 2021-11-13 14:59:30Z

2

What you want is only possible if the expectedKeys is static - if you hard-code it. (If they're dynamically generated and not present in the source code, it's impossible, because TypeScript types only operate on what exists at compile-time - at runtime, you're only left with plain JavaScript)

Define the array as const so it doesn't get widened to string[], and then you can make a Record composed of the keys (the array items) and string.

const expectedKeys = ['timestamp', 'price'] as const;
type ExpectedType = Record<
    typeof expectedKeys[number],
    string
>;

answered Nov 13, 2021 at 14:59

CertainPerformance

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