TypeScript: Extract Nested Object Keys as Dot-Notation Strings (With and Without Infer)

nestedDotNotationPaths.ts

/**
 * Recursively extracts nested keys of an object as dot-notation strings.
 * Useful for representing nested object paths in a string format.
 *
 * Example:
 * type Theme = { colors: { primary: string; secondary: { light: string; dark: string } } };
 * type Paths = NestedDotNotationPaths<Theme['colors']>;
 * "primary" | "secondary.light" | "secondary.dark"
 */
export type NestedDotNotationPaths<T> = T extends object
  ? {
      [K in keyof T & (string | number)]: T[K] extends Record<
        string | number,
        unknown
      >
        ? `${K}.${NestedDotNotationPaths<T[K]>}` // Recursive case: if the value is an object, dive deeper
        : T[K] extends string
        ? `${K}` // Base case: if the value is a string, return the key
        : never; // Invalid case: neither string nor object
    }[keyof T & (string | number)]
  : never;

/**
 * Recursively extracts nested keys of an object as dot-notation strings using `infer`.
 * Useful for representing nested object paths in a string format.
 *
 * Example:
 * type Theme = { colors: { primary: string; secondary: { light: string; dark: string } } };
 * type Paths = NestedDotNotationPathsInfer<Theme['colors']>;
 * "primary" | "secondary.light" | "secondary.dark"
 */
export type NestedDotNotationPathsInfer<T> = T extends object
  ? {
      [K in keyof T & (string | number)]: T[K] extends infer V
        ? V extends Record<string | number, unknown>
          ? `${K}.${NestedDotNotationPathsInfer<V>}` // Recursive case: if the value is an object, dive deeper
          : V extends string
          ? `${K}` // Base case: if the value is a string, return the key
          : never // Invalid case: neither string nor object
        : never; // Invalid case: key is not a string or number
    }[keyof T & (string | number)]
  : never;

Two utility types for recursively extracting nested object keys as dot-notation strings:

1. Direct Access: No infer, simpler structure.
2. Infer Version: Uses infer for value type extraction.

Example:

type Theme = {
  colors: {
    primary: string;
    secondary: { light: string; dark: string };
  };
};

type Paths = NestedDotNotationPaths<Theme['colors']>; 
// "primary" | "secondary.light" | "secondary.dark"

type PathsInfer = NestedDotNotationPathsInfer<Theme['colors']>; 
// "primary" | "secondary.light" | "secondary.dark"