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I am trying to build a required object parameter depending on which path you target in a object

type Split<S extends string, D extends string> =
    string extends S ? string[] :
    S extends '' ? [] :
    S extends `${infer T}${D}${infer U}` ? [T, ...Split<U, D>] :
    [S];

type PropType<T, Path extends string> =
    string extends Path ? unknown :
    Path extends keyof T ? T[Path] :
    Path extends `${infer K}.${infer R}` ? K extends keyof T ? PropType<T[K], R> : unknown :
    unknown; 

type ParseMustaches_<T extends string> = 
  T extends `${infer U}{{${infer V}}}${infer W}` 
    ? Record<V, string> 
    : never

type ParseMustaches<T extends string[]> = ParseMustaches_<T[number]>;

type Prop<T, K extends keyof T> = T[K];

declare function translate<T extends { [L in K]: string }, K extends string>(obj: T, path: K, placeholders: ParseMustaches<Split<T[K], " ">>): void;

const obj = {
  "title": "Welcome to {{sitename}}, {{user}}",
  "button": {
    "text": "Click here to go to {{location}}",
    "num": 5
  }
} as const;

translate(obj, "title", { sitename: "", user: "" }) // works
translate(obj, "button.text", { }) // does not work

It seems to work for top level properties but nested properties fail, how could I fix this? link to playground

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1 Answer 1

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The problem lies in the part where you said T extends { [L in K]: string }, since K here is not a key in general, but a path. So you need a little extra work to make your magic happen.

type Split<S extends string, D extends string> =
  string extends S ? string[] :
  S extends '' ? [] :
  S extends `${infer T}${D}${infer U}` ? [T, ...Split<U, D>] :
  [S];

type PropType<T, Path extends string> =
  string extends Path ? unknown :
  Path extends keyof T ? T[Path] :
  Path extends `${infer K}.${infer R}` ? K extends keyof T ? PropType<T[K], R> : unknown :
  unknown;

type ParseMustaches_<T extends string> =
  T extends `${infer U}{{${infer V}}}${infer W}`
  ? Record<V, string>
  : never

type ParseMustaches<T extends string[]> = ParseMustaches_<T[number]>;

// You need to this utility type
type TypeOnPath<Path extends string, T> =
  string extends Path ? unknown :
  Path extends `${infer K}.${infer R}` ? { [k in K]: TypeOnPath<R, T> } :
  { [k in Path]: T };

declare function translate<K extends string, T extends TypeOnPath<K, string>>
  (obj: T, path: K, placeholders: ParseMustaches<Split<PropType<T, K> & string, " ">>): void;

const obj = {
  "title": "Welcome to {{sitename}}, {{user}}",
  "button": {
    "text": "Click here to go to {{location}}",
    "num": 5
  }
} as const;

translate(obj, "title", { sitename: "", user: "" })
translate(obj, "button.text", {}) // now we have an error saying that 'location' is missing
translate(obj, "button.num", {}) // correctly report error saying 'button.num' is not a string

See this Playground Link

Update

To get the desired behavior mentioned in the comment, you can modify Split:

// Remove constraint on S so that it takes any input type
type Split<S, D extends string> =
  S extends string ? (
    string extends S ? string[] :
    S extends '' ? [] :
    S extends `${infer T}${D}${infer U}` ? [T, ...Split<U, D>] :
    [S]
  ) : never;

type PropType<T, Path extends string> =
  string extends Path ? unknown :
  Path extends keyof T ? T[Path] :
  Path extends `${infer K}.${infer R}` ? K extends keyof T ? PropType<T[K], R> : unknown :
  unknown;

type ParseMustaches_<T extends string> =
  T extends `${infer U}{{${infer V}}}${infer W}`
  ? Record<V, string>
  : never

type ParseMustaches<T extends string[]> = ParseMustaches_<T[number]>;

type TypeOnPath<Path extends string, T> =
  string extends Path ? unknown :
  Path extends `${infer K}.${infer R}` ? { [k in K]: TypeOnPath<R, T> } :
  { [k in Path]: T };

// This time we use TypeOnPath<K, any>
declare function translate<K extends string, T extends TypeOnPath<K, any>>
  (obj: T, path: K, placeholders: ParseMustaches<Split<PropType<T, K>, " ">>): void;

const obj = {
  "title": "Welcome to {{sitename}}, {{user}}",
  "button": {
    "text": "Click here to go to {{location}}",
    "num": 5
  }
} as const;

translate(obj, "title", { sitename: "", user: "" })
translate(obj, "button.text", {}) // now we have an error saying that 'location' is missing
translate(obj, "button.num", {}) // error with never

See this Playground Link

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3 Comments

Thank you! but that last error is not wanted behaviour, the path can resolve to any value in the object, it's fine if it's a number or string or array
But how are you supposed to parse the value then? Your ParseMustaches_ only accepts string.
If the value resolves to something that's not a string, the 3rd param should be never, just like when there are no mustache notations

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