From ecedb2e02451fb18ade5888d24e09956754accea Mon Sep 17 00:00:00 2001
From: ersimont <8042088+ersimont@users.noreply.github.com>
Date: Tue, 24 Nov 2020 21:04:29 -0500
Subject: [PATCH] feat(micro-dash): improve the typing of `pick()`

---
 .../micro-dash/src/lib/object/pick.spec.ts    |  6 ---
 projects/micro-dash/src/lib/object/pick.ts    | 12 ++---
 .../src/typing-tests/object/pick.dts-spec.ts  | 54 +++++++++++++++++++
 3 files changed, 60 insertions(+), 12 deletions(-)
 create mode 100644 projects/micro-dash/src/typing-tests/object/pick.dts-spec.ts

diff --git a/projects/micro-dash/src/lib/object/pick.spec.ts b/projects/micro-dash/src/lib/object/pick.spec.ts
index cbb5a85a..767d83fc 100644
--- a/projects/micro-dash/src/lib/object/pick.spec.ts
+++ b/projects/micro-dash/src/lib/object/pick.spec.ts
@@ -1,12 +1,6 @@
 import { pick } from './pick';
 
 describe('pick()', () => {
-  it('fancily narrows types', () => {
-    let picked: { a: number; b: string };
-    picked = pick({ a: 1, b: 'b', c: { d: 3 } }, 'a', 'b');
-    expect(picked).toEqual({ a: 1, b: 'b' });
-  });
-
   //
   // stolen from https://github.com/healthiers/mini-dash
   //
diff --git a/projects/micro-dash/src/lib/object/pick.ts b/projects/micro-dash/src/lib/object/pick.ts
index 49320c75..1da291ed 100644
--- a/projects/micro-dash/src/lib/object/pick.ts
+++ b/projects/micro-dash/src/lib/object/pick.ts
@@ -1,4 +1,4 @@
-import { Nil } from '../interfaces';
+import { IfCouldBe, Nil } from '../interfaces';
 
 /**
  * Creates an object composed of the picked `object` properties.
@@ -10,14 +10,14 @@ import { Nil } from '../interfaces';
  * - Lodash: 7,718 bytes
  * - Micro-dash: 142 bytes
  */
-export function pick<T, K extends keyof T>(
-  object: T | Nil,
-  ...paths: K[]
-): T extends Nil ? {} : Pick<T, K> {
+export function pick<T, P extends Array<keyof NonNullable<T>>>(
+  object: T,
+  ...paths: P
+): { [K in P[number]]: NonNullable<T>[K] } | IfCouldBe<T, Nil, {}> {
   const result: any = {};
   if (object != null) {
     for (const path of paths) {
-      result[path] = object[path];
+      result[path] = object[path as keyof T];
     }
   }
   return result;
diff --git a/projects/micro-dash/src/typing-tests/object/pick.dts-spec.ts b/projects/micro-dash/src/typing-tests/object/pick.dts-spec.ts
new file mode 100644
index 00000000..22cffdd6
--- /dev/null
+++ b/projects/micro-dash/src/typing-tests/object/pick.dts-spec.ts
@@ -0,0 +1,54 @@
+import { pick } from '../../lib/object';
+
+declare const str: string;
+declare const num: number;
+
+declare const obj: {
+  a: number;
+  b: Date;
+};
+// $ExpectType { b: Date; }
+pick(obj, 'b');
+
+declare const objOrNull: typeof obj | null;
+declare const objOrUndefined: typeof obj | undefined;
+declare const objOrNil: typeof obj | null | undefined;
+// $ExpectType {} | { b: Date; }
+pick(objOrNull, 'b');
+// $ExpectType {} | { b: Date; }
+pick(objOrUndefined, 'b');
+// $ExpectType {} | { b: Date; }
+pick(objOrNil, 'b');
+
+declare const indexed: {
+  [k: string]: number;
+  [k: number]: number;
+};
+// $ExpectType { hi: number; }
+pick(indexed, 'hi');
+// $ExpectType { 5: number; }
+pick(indexed, 5);
+// $ExpectType { hi: number; 5: number; }
+pick(indexed, 'hi', 5);
+// $ExpectType { [x: string]: number; }
+pick(indexed, str);
+
+declare const record: Record<string, number>;
+// $ExpectType { [x: string]: number; }
+pick(record, str);
+
+declare const composite: {
+  [k: number]: Date;
+  a: 'eh';
+  b: 'bee';
+};
+// $ExpectType { a: "eh"; }
+pick(composite, 'a');
+// $ExpectType { a: "eh"; b: "bee"; }
+pick(composite, 'a', 'b');
+// $ExpectType { 1: Date; }
+pick(composite, 1);
+// $ExpectType { [x: number]: Date; }
+pick(composite, num);
+// $ExpectType { [x: number]: Date; a: "eh"; }
+pick(composite, num, 'a');