Typescript mapped generics

November 5, 2021

I’m trying to get TS generics to map to a new object. In short, I’m trying to convert:

{ 
  key: { handler: () => string },
  key2: { hander: () => number },
}

to:

{ key: string, key2: number }

full example:

type programOption = {
  validator: () => unknown
}

type programConfig<T extends {[key: string]: programOption} = {}> = {
  options: T,
  handler: (data: mapProgramConfig<T>) => void,
}

type mapProgramConfig<T extends {[key: string]: programOption}> = {
  [K in keyof T]: ReturnType<programOption['validator']>
}

type mapProgramConfigHardcoded<T> = {
  fruit: string,
  animal: number
}

class Program {
  constructor (config: programConfig) {}
}

const foo = new Program({
  options: {
    'fruit': { validator: () => 'asdf' },
    'animal': { validator: () => 42 },
  },
  handler: ({fruit, animal, thing}) => {

  },
});

Exactly what I’m trying to do can be seen if you replace mapProgramConfig with mapProgramConfigHardcoded in the programConfig type, but I can’t seem to make it work in the generic case.

ts playground link

>Solution :

Consider this solution:

type ProgramOption<T> = {
  validator?: () => T
}


type Convert<Obj extends Record<string, ProgramOption<any>>> = {
  [Prop in keyof Obj]: Obj[Prop]['validator'] extends () => infer Return ? Return : never
}
const program = <
  Keys extends PropertyKey,
  ValidatorValues extends string | number,
  Options extends Record<Keys, ProgramOption<ValidatorValues>>,
  Handler extends (data: Convert<Options>) => void,
  >(data: { options: Options, handler: Handler },) => {
  return data
}

const foo = program({
  options: {
    'fruit': { validator: () => 'string' },
    'animal': { validator: () => 42 },
  },
  handler: (obj) => {
    obj.animal // 42
    obj.fruit // 'string'
  }
});

Playground

In order to infer obj argument in handler property you need infer all nested keys and values.

Keys – refers to fruit and animal keys of nested object
ValidatorValues – refers to Validator return type
Options – refers to whole options property
Handler – refers to handler accordingly.