Introduce some metaprogramming types

src/functional.ts

@@ -1,18 +1,20 @@
+import { LeafElems } from "./metaprogramming";
+
 /**
  * Flattens an object so that the return value's keys are the path
  * to a value in the source object. E.g. flattenObject({the: {answer: 42}})
  * returns {"the.answser": 42}
  * @param obj An object to be flattened
  * @return An array where values come from obj and keys are the path in obj to that value.
  */
-export function* flattenObject(obj: Record<string, unknown>): Generator<[string, unknown]> {
-  function* helper(path: string[], obj: Record<string, unknown>): Generator<[string, unknown]> {
+export function* flattenObject<T extends object>(obj: T): Generator<[string, unknown]> {
+  function* helper<V extends object>(path: string[], obj: V): Generator<[string, unknown]> {
     for (const [k, v] of Object.entries(obj)) {
       if (typeof v !== "object" || v === null) {
         yield [[...path, k].join("."), v];
       } else {
         // Object.entries loses type info, so we must cast
-        yield* helper([...path, k], v as Record<string, unknown>);
+        yield* helper([...path, k], v);
       }
     }
   }
@@ -25,41 +27,43 @@ export function* flattenObject(obj: Record<string, unknown>): Generator<[string,
  * [...flatten([[[1]], [2], 3])] = [1, 2, 3]
  */
 // eslint-disable-next-line  @typescript-eslint/no-explicit-any
-export function* flattenArray<T = any>(arr: unknown[]): Generator<T> {
+export function* flattenArray<T extends unknown[]>(arr: T): Generator<LeafElems<T>> {
   for (const val of arr) {
     if (Array.isArray(val)) {
       yield* flattenArray(val);
     } else {
-      yield val as T;
+      yield val as LeafElems<T>;
     }
   }
 }
 
 /** Shorthand for flattenObject. */
-export function flatten(obj: Record<string, unknown>): Generator<[string, unknown]>;
+export function flatten<T extends object>(obj: T): Generator<[string, string]>;
 /** Shorthand for flattenArray. */
 // eslint-disable-next-line @typescript-eslint/no-explicit-any
-export function flatten<T = any>(arr: unknown[]): Generator<T>;
+export function flatten<T extends unknown[]>(arr: T): Generator<LeafElems<T>>;
 
 /** Flattens an object or array. */
-export function flatten<T>(
-  objOrArr: Record<string, unknown> | unknown[]
-): Generator<[string, unknown]> | Generator<T> {
+export function flatten<T extends unknown[] | object>(objOrArr: T): unknown {
   if (Array.isArray(objOrArr)) {
-    return flattenArray<T>(objOrArr);
+    return flattenArray(objOrArr);
   } else {
     return flattenObject(objOrArr);
   }
 }
 
+type RecursiveElems<T extends unknown[]> = {
+  [Key in keyof T]: T[Key] extends unknown[] ? T[Key] | RecursiveElems<T[Key]> : T[Key];
+}[number];
+
 /**
  * Used with reduce to flatten in place.
  * Due to the quirks of TypeScript, callers must pass [] as the
  * second argument to reduce.
  */
 // eslint-disable-next-line @typescript-eslint/no-explicit-any
 export function reduceFlat<T = any>(accum: T[] | undefined, next: unknown): T[] {
-  return [...(accum || []), ...flatten<T>([next])];
+  return [...(accum || []), ...(flatten([next]) as Generator<T>)];
 }
 
 /**

src/metaprogramming.ts

@@ -0,0 +1,83 @@
+type Primitive = string | number | boolean | Function;
+
+/**
+ * RecursiveKeyOf is a type for keys of an objet usind dots for subfields.
+ * For a given object: {a: {b: {c: number}}, d } the RecursiveKeysOf are
+ * 'a' | 'a.b' | 'a.b.c' | 'd'
+ */
+export type RecursiveKeyOf<T> = T extends Primitive
+  ? never
+  :
+      | (keyof T & string)
+      | {
+          [P in keyof T & string]: RecursiveSubKeys<T, P>;
+        }[keyof T & string];
+
+type RecursiveSubKeys<T, P extends keyof T & string> = T[P] extends (infer Elem)[]
+  ? `${P}.${RecursiveKeyOf<Elem>}`
+  : T[P] extends object
+  ? `${P}.${RecursiveKeyOf<T[P]>}`
+  : never;
+
+/**
+ * LeafKeysOf is like RecursiveKeysOf but omits the keys for any object.
+ * For a given object: {a: {b: {c: number}}, d } the LeafKeysOf are
+ * 'a.b.c' | 'd'
+ */
+export type LeafKeysOf<T extends object> = {
+  [Key in keyof T & (string | number)]: T[Key] extends unknown[]
+    ? `${Key}`
+    : T[Key] extends object
+    ? `${Key}.${RecursiveKeyOf<T[Key]>}`
+    : `${Key}`;
+}[keyof T & (string | number)];
+
+/**
+ * SameType is used in testing to verify that two types are the same.
+ * Usage:
+ * const test: SameType<A, B> = true.
+ * The assigment will fail if the types are different.
+ */
+export type SameType<T, V> = T extends V ? (V extends T ? true : false) : false;
+
+type HeadOf<T extends string> = [T extends `${infer Head}.${infer Tail}` ? Head : T][number];
+
+type TailsOf<T extends string, Head extends string> = [
+  T extends `${Head}.${infer Tail}` ? Tail : never
+][number];
+
+/**
+ * DeepOmit allows you to omit fields from a nested structure using recursive keys.
+ */
+export type DeepOmit<T extends object, Keys extends RecursiveKeyOf<T>> = DeepOmitUnsafe<T, Keys>;
+
+type DeepOmitUnsafe<T, Keys extends string> = {
+  [Key in Exclude<keyof T, Keys>]: Key extends Keys
+    ? T[Key] | undefined
+    : Key extends HeadOf<Keys>
+    ? DeepOmitUnsafe<T[Key], TailsOf<Keys, Key>>
+    : T[Key];
+};
+
+export type DeepPick<T extends object, Keys extends RecursiveKeyOf<T>> = DeepPickUnsafe<T, Keys>;
+
+type DeepPickUnsafe<T, Keys extends string> = {
+  [Key in Extract<keyof T, HeadOf<Keys>>]: Key extends Keys
+    ? T[Key]
+    : DeepPickUnsafe<T[Key], TailsOf<Keys, Key>>;
+};
+
+/** In the array LeafElems<[[["a"], "b"], ["c"]]> is "a" | "b" | "c" */
+export type LeafElems<T> = T extends Array<infer Elem>
+  ? Elem extends unknown[]
+    ? LeafElems<Elem>
+    : Elem
+  : T;
+
+/**
+ * In the object {a: number, b: { c: string } },
+ * LeafValues is number | string
+ */
+export type LeafValues<T extends object> = {
+  [Key in keyof T]: T[Key] extends object ? LeafValues<T[Key]> : T[Key];
+}[keyof T];

src/test/functional.spec.ts

@@ -1,5 +1,6 @@
 import { expect } from "chai";
 import { flatten } from "lodash";
+import { SameType } from "../metaprogramming";
 
 import * as f from "../functional";
 
@@ -13,6 +14,13 @@ describe("functional", () => {
       expect([...f.flatten({ a: "b" })]).to.deep.equal([["a", "b"]]);
     });
 
+    it("Gets the right type for flattening arrays", () => {
+      const arr = [[["a"], "b"], ["c"]];
+      const flattened = [...f.flattenArray(arr)];
+      const test: SameType<typeof flattened, string[]> = true;
+      expect(test).to.be.true;
+    });
+
     it("can handle nested objects", () => {
       const init = {
         outer: {

src/test/metapgrogramming.spec.ts

@@ -0,0 +1,76 @@
+import { expect } from "chai";
+import { SameType, RecursiveKeyOf, LeafElems, DeepPick, DeepOmit } from "../metaprogramming";
+
+describe("metaprogramming", () => {
+  it("can calcluate recursive keys", () => {
+    const test: SameType<
+      RecursiveKeyOf<{
+        a: number;
+        b: {
+          c: boolean;
+          d: {
+            e: number;
+          };
+        };
+      }>,
+      "a" | "a.b" | "a.b.c" | "a.b.d" | "a.b.d.e"
+    > = true;
+    expect(test).to.be.true;
+  });
+
+  it("can detect recursive elems", () => {
+    const test: SameType<LeafElems<[[["a"], "b"], ["c"]]>, "a" | "b" | "c"> = true;
+    expect(test).to.be.true;
+  });
+
+  it("Can deep pick", () => {
+    interface original {
+      a: number;
+      b: {
+        c: boolean;
+        d: {
+          e: number;
+        };
+        g: boolean;
+      };
+      h: number;
+    }
+
+    interface expected {
+      a: number;
+      b: {
+        c: boolean;
+      };
+    }
+
+    const test: SameType<DeepPick<original, "a" | "b.c">, expected> = true;
+    expect(test).to.be.true;
+  });
+
+  it("can deep omit", () => {
+    interface original {
+      a: number;
+      b: {
+        c: boolean;
+        d: {
+          e: number;
+        };
+        g: boolean;
+      };
+      h: number;
+    }
+
+    interface expected {
+      b: {
+        d: {
+          e: number;
+        };
+        g: boolean;
+      };
+      h: number;
+    }
+
+    const test: SameType<DeepOmit<original, "a" | "b.c">, expected> = true;
+    expect(test).to.be.true;
+  });
+});