[fix] Nullish case in runtime #6

[optimize] add Mutation notice in ReadMe

.gitignore

@@ -1,3 +1,4 @@
 node_modules/
 package-lock.json
 dist/
+.vscode/

ReadMe.md

@@ -20,6 +20,11 @@ While `Array.prototype.unique()` invoked with:
 
 3.  one **function** parameter, it'll call this function for each array element, and then deduplicates the origin array based on these _returned values_.
 
+Notice:
+
+-   the **Returned value** is a new array, no mutation happens in the original array
+-   **Empty/nullish items** are treated as nullish values
+
 ## Typical cases
 
 ```JavaScript

package.json

@@ -1,6 +1,6 @@
 {
     "name": "array-unique-proposal",
-    "version": "0.2.0",
+    "version": "0.2.1",
     "description": "ECMAScript proposal for Deduplicating method of Array",
     "keywords": [
         "JavaScript",
@@ -29,13 +29,13 @@
         "prepublishOnly": "npm test  &&  npm run build"
     },
     "devDependencies": {
-        "@types/jest": "^26.0.0",
+        "@types/jest": "^26.0.4",
         "husky": "^4.2.5",
-        "jest": "^26.0.1",
-        "lint-staged": "^10.2.9",
+        "jest": "^26.1.0",
+        "lint-staged": "^10.2.11",
         "prettier": "^2.0.5",
-        "ts-jest": "^26.1.0",
-        "typescript": "^3.9.5"
+        "ts-jest": "^26.1.2",
+        "typescript": "^3.9.6"
     },
     "prettier": {
         "singleQuote": true,

polyfill/index.spec.ts

@@ -62,4 +62,12 @@ describe('Array.prototype.unique()', () => {
             origin
         );
     });
+
+    it('should treat an empty/nullish item as a nullish value', () => {
+        expect([1, , 2, undefined, null, 1].unique()).toEqual([1, , 2, null]);
+
+        expect(
+            [{ id: 1 }, , { id: 2 }, undefined, null, { id: 1 }].unique('id')
+        ).toEqual([{ id: 1 }, , { id: 2 }, null]);
+    });
 });

polyfill/index.ts

@@ -1,6 +1,8 @@
 type Resolver<T> = (item: T) => any;
 
-type ValueResolver<T> = number | string | symbol | Resolver<T>;
+type Indexer<T> = number | keyof T | symbol;
+
+type ValueResolver<T> = Indexer<T> | Resolver<T>;
 
 interface Array<T> {
     unique(valueResolver?: ValueResolver<T>): T[];
@@ -16,7 +18,9 @@ if (typeof Array.prototype.unique !== 'function')
             const key = typeof valueResolver !== 'function' && valueResolver,
                 map = new Map<any, T>();
 
-            valueResolver = key ? item => item[key] : valueResolver;
+            valueResolver = key
+                ? (item: Record<Indexer<T>, any>) => item?.[key] ?? item
+                : valueResolver;
 
             for (const item of this)
                 map.set((valueResolver as Resolver<T>)(item), item);