get.ts

type GetIndexedField<T, K> = K extends keyof T
  ? T[K]
  : K extends `${number}`
    ? '0' extends keyof T
      ? undefined
      : number extends keyof T
        ? T[number]
        : undefined
    : undefined

type FieldWithPossiblyUndefined<T, Key> =
  | GetFieldType<Exclude<T, undefined>, Key>
  | Extract<T, undefined>

type IndexedFieldWithPossiblyUndefined<T, Key> =
  | GetIndexedField<Exclude<T, undefined>, Key>
  | Extract<T, undefined>

export type GetFieldType<T, P> = P extends `${infer Left}.${infer Right}`
  ? Left extends keyof T
    ? FieldWithPossiblyUndefined<T[Left], Right>
    : Left extends `${infer FieldKey}[${infer IndexKey}]`
      ? FieldKey extends keyof T
        ? FieldWithPossiblyUndefined<
            IndexedFieldWithPossiblyUndefined<T[FieldKey], IndexKey>,
            Right
          >
        : undefined
      : undefined
  : P extends keyof T
    ? T[P]
    : P extends `${infer FieldKey}[${infer IndexKey}]`
      ? FieldKey extends keyof T
        ? IndexedFieldWithPossiblyUndefined<T[FieldKey], IndexKey>
        : undefined
      : undefined

export default function get<
  TPath extends string,
  TData,
  TDefault = GetFieldType<TData, TPath>,
>(
  path: TPath,
  data: TData,
  defaultValue?: TDefault,
): GetFieldType<TData, TPath> | TDefault {
  const value = path
    .split(/[.[\]]/)
    .filter(Boolean)
    .reduce<GetFieldType<TData, TPath>>(
      (value, key) => (value as any)?.[key],
      data as any,
    )

  return value !== undefined ? value : (defaultValue as TDefault)
}