Safely pick nested properties from object using dot-path.

Nested Pick (Typescript)

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 function _get<TData, TPath extends string, TDefault = GetFieldType<TData, TPath>>(
	data: TData,
	path: TPath,
	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);
}