Infer types from an object. Make object-key prefixed with "_" partials (not required)

defaults and types.ts

// Keys in R overwrite the ones in Default.
type Modify<Default, R> = Omit<Default, keyof R> & R

// get only types where key is prefixed with Prefix.
type FilterPrefixed<O, Prefix extends string> = {
  [K in keyof O as K extends `${Prefix}${infer _}` ? K : never]: O[K]
}

/**
 * Object-Keys that are not prefixed with "_" become partials.
 * Example: a      = { b: 1, _c: 2 }.
 *          type A = _Partial<typeof a> // { b?: number; c: number }
 */
type _Partial<O> = Modify<Partial<O>, FilterPrefixed<O, "_">>

const a = { b: 1, _c: 2 }
type A = _Partial<typeof a>