filtering types from an interface or type using conditionals.

filter.type.ts

type Job = {
  id: string;
  salary: number;
  title: string;
}

interface User {
  id: string;
  name: string;
  company: string;
  position: 'internship' | 'regular';
  mobileNumber: number;
  gender: string;
  jobs: Job[];

  getAge: () => number;
}

type OnlyStringForm = {
  [key in keyof User]: User[key] extends string ? key : never
}[keyof User]

type OnlyNumberString = {
  [key in keyof User]: User[key] extends number ? key : never
}[keyof User]

type OnlyMethodsThatReturnNumber = {
  [key in keyof User]: User[key] extends () => number ? key : never
}[keyof User]


type Filtered<ObjectType, Condition> = {
  [key in keyof ObjectType]: ObjectType[key] extends Condition ? key : never
}[keyof ObjectType]

type UsingFilteredOnlyStringForm = Filtered<User, string> // "id" | "name" | "company" | "position" | "gender"
type UsingFilteredOnlyNumberForm = Filtered<User, number> // "mobileNumber"
type UsingFilteredOnlyMethodsThatReturnNumber = Filtered<User, () => number> // "getAge"