Introduce createEnumSubset method on EnumWrapper

docs/EnumWrapper.md

@@ -70,6 +70,8 @@ Usage documentation for the `EnumWrapper` portion of `ts-enum-util`.
     -   [Lookup Value by Key](#lookup-value-by-key)
         -   [EnumWrapper.prototype.getValueOrThrow](#enumwrapperprototypegetvalueorthrow)
         -   [EnumWrapper.prototype.getValueOrDefault](#enumwrapperprototypegetvalueordefault)
+    -   [Create a new enum-like object with a subset of an enum's entries](#create-a-new-enum-like-object-with-a-subset-of-an-enums-entries)
+        -   [EnumWrapper.prototype.createEnumSubset](#enumwrapperprototypecreateenumsubset)
 
 <!-- /TOC -->
 
@@ -819,3 +821,30 @@ EnumWrapper.prototype.getValueOrDefault(
     defaultValue?: EnumType | ValueType
 ): EnumType | ValueType | undefined
 ```
+
+### Create a new enum-like object with a subset of an enum's entries
+
+#### EnumWrapper.prototype.createEnumSubset
+
+Use this method to create a new well-typed [Enum-Like Object](#enum-like-object) that contains
+a subset of entries from the original enum.
+
+```ts
+EnumWrapper.prototype.createEnumSubset(
+    ...keys: readonly KeyType[]
+): EnumLike
+```
+
+Example:
+
+```ts
+enum Numbers {
+    One = 1,
+    Two = 2,
+    Three = 3,
+    Four = 4
+}
+
+const EvenNumbers = $enum(Numbers).createEnumSubset("Two", "Four");
+type EvenNumbers = typeof EvenNumbers[keyof typeof EvenNumbers];
+```

src/EnumWrapper.ts

@@ -679,6 +679,21 @@ export class EnumWrapper<
             return defaultValue;
         }
     }
+
+    /**
+     * Creates an enum-like object that contains a subset of entries of the
+     * original enum.
+     * @param keys - The keys to include in the new enum-like object.
+     * @returns A new enum-like object.
+     */
+    public createEnumSubset<K extends StringKeyOf<T>>(
+        ...keys: readonly K[]
+    ): Pick<T, K> {
+        return keys.reduce<Record<string, T[K]>>((result, key) => {
+            result[key] = this.enumObj[key];
+            return result;
+        }, {}) as Pick<T, K>;
+    }
 }
 
 // HACK: Forcefully overriding the value of the [Symbol.toStringTag] property.

tests/EnumWrapper-mixed.test.ts

@@ -542,4 +542,11 @@ describe("EnumWrapper: number+string enum", () => {
         );
         expect(enumWrapper.getValueOrDefault(undefined, -1)).toBe(-1);
     });
+
+    test("createEnumSubset()", () => {
+        expect(enumWrapper.createEnumSubset("A", "C")).toEqual({
+            A: TestEnum.A,
+            C: TestEnum.C
+        });
+    });
 });

tests/EnumWrapper-number.test.ts

@@ -524,4 +524,11 @@ describe("EnumWrapper: string enum", () => {
         );
         expect(enumWrapper.getValueOrDefault(undefined, -1)).toBe(-1);
     });
+
+    test("createEnumSubset()", () => {
+        expect(enumWrapper.createEnumSubset("A", "C")).toEqual({
+            A: TestEnum.A,
+            C: TestEnum.C
+        });
+    });
 });

tests/EnumWrapper-string.test.ts

@@ -531,4 +531,11 @@ describe("EnumWrapper: string enum", () => {
         );
         expect(enumWrapper.getValueOrDefault(undefined, "foo")).toBe("foo");
     });
+
+    test("createEnumSubset()", () => {
+        expect(enumWrapper.createEnumSubset("A", "C")).toEqual({
+            A: TestEnum.A,
+            C: TestEnum.C
+        });
+    });
 });

type_tests/v2_9_plus/EnumWrapper/enum-mixed.dtslint.ts

@@ -266,3 +266,6 @@ enumWrapper.getValueOrDefault(str, str);
 enumWrapper.getValueOrDefault(str, numstr);
 // $ExpectType string | number | undefined
 enumWrapper.getValueOrDefault(str, numstrOrUndefined);
+
+// $ExpectType Pick<typeof TestEnum, "A" | "C">
+enumWrapper.createEnumSubset("A", "C");

type_tests/v2_9_plus/EnumWrapper/enum-number.dtslint.ts

@@ -268,3 +268,6 @@ enumWrapper.getValueOrDefault(str, num);
 enumWrapper.getValueOrDefault(str, numOrUndefined);
 // $ExpectError
 enumWrapper.getValueOrDefault(str, str);
+
+// $ExpectType Pick<typeof TestEnum, "A" | "C">
+enumWrapper.createEnumSubset("A", "C");

type_tests/v2_9_plus/EnumWrapper/enum-string.dtslint.ts

@@ -268,3 +268,6 @@ enumWrapper.getValueOrDefault(str, str);
 enumWrapper.getValueOrDefault(str, strOrUndefined);
 // $ExpectError
 enumWrapper.getValueOrDefault(str, num);
+
+// $ExpectType Pick<typeof TestEnum, "A" | "C">
+enumWrapper.createEnumSubset("A", "C");