Typescript mapping keys to uppercase

Question

I have the following object:

const foo = {
  fieldAnt: "foo",
  fieldCab: "baz"
};

I'd like to automatically map that to a type with the keys made to uppercase and underscored:

type FIELDS = {
  FIELD_ANT: "fieldAnt",
  FIELD_CAB: "fieldCab" 
}

Is it possible using keyof or type mapping to programmatically achieve this with typescript?

Typescript only provides types, you're kinda asking that It actually affects and changes objects. — Elias
– Elias, Commented Mar 19, 2020 at 18:47
unfortunately you cannot do string manipulation using typescript types so while you could enforce the FIELDS values being equal to the keyof foo. You could not created the required uppercase, underscored keys themselves. — teddybeard
– teddybeard, Commented Mar 19, 2020 at 18:57
The idea is that I would run a function to actually create that object at runtime, but need the types present for static checks — Omair Vaiyani
– Omair Vaiyani, Commented Mar 20, 2020 at 9:01

Buszmen · Accepted Answer · 2021-09-03 08:40:39Z

12

A little late :P but, with a little help I was able to do it:

const foo = {
  fieldAnt: "foo",
  fieldCab: "baz"
};

type CamelToSnakeCase<S extends string> =
  S extends `${infer T}${infer U}` ?
  `${T extends Capitalize<T> ? "_" : ""}${Lowercase<T>}${CamelToSnakeCase<U>}` :
  S

type Convert<T> = { [P in keyof T as Uppercase<CamelToSnakeCase<string & P>>]: P }

type Fields = Convert<typeof foo>
// type Fields = {
//  FIELD_ANT: "fieldAnt",
//  FIELD_CAB: "fieldCab" 
// }

TS Playground

answered Sep 3, 2021 at 8:40

Buszmen

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