feat: enum support (#5)

README.md

@@ -138,3 +138,28 @@ import { Decoder, array, field, number, string } from "json-decode";
 array(number)([1, 2, 3]); // [1, 2, 3]
 array(number)([1, 2, '3']); // throws a DecodeError
 ```
+
+### Decoding a TypeScript enum 
+
+```typescript
+import { enumerator } from "json-decode";
+
+enum Choice {
+  carrot = 'carrot',
+  stick = 'stick'
+}
+
+enumerator(Choice)('carrot'); // Choice.carrot
+enumerator(Choice)('stick'); // Choice.stick
+enumerator(Choice)('banana'); // throws a DecodeError
+
+enum Fruits {
+  apple, 
+  banana,
+  pear,
+}
+
+enumerator(Fruits)(0); // Fruits.apple
+enumerator(Fruits)(1) // Fruits.banana
+enumerator(Fruits)(3) // throws a DecodeError
+```

src/index.d.ts

@@ -25,3 +25,5 @@ export function nullable<T>(decoder: Decoder<T>): Decoder<T | null>;
 export function optional<T>(decoder: Decoder<T>): Decoder<T | undefined>;
 
 export const string: Decoder<string>;
+
+export const enumerator: <T>(enumObject: T) => Decoder<T[keyof T]>;

src/index.ts

@@ -4,11 +4,12 @@ export {
   bool,
   Decoder,
   DecoderError,
+  enumerator,
   int,
   field,
   number,
   float,
   nullable,
   optional,
-  string
+  string,
 } from './lib/json-decode'

src/lib/json-decode.spec.ts

@@ -4,15 +4,15 @@ import {
   bigint,
   bool,
   Decoder,
-  DecoderError,
+  DecoderError, enumerator,
   field,
   float,
   int,
   nullable,
   number,
   optional,
-  string,
-} from './json-decode';
+  string
+} from "./json-decode";
 
 describe('bool', () => {
   it('decodes a boolean', () => {
@@ -125,6 +125,55 @@ describe('nullable', () => {
   });
 });
 
+describe('enumerator', () => {
+
+  describe('when the enum is a string enum', () => {
+    enum Choice {
+      carrot = 'carrot',
+      stick = 'stick'
+    }
+
+    it('decodes the enum', () => {
+      expect(enumerator(Choice)('carrot')).toEqual(Choice.carrot);
+    });
+
+    it('throws when the value is not a member of the enum', () => {
+      expect(() => enumerator(Choice)('banana')).toThrowError(DecoderError);
+    });
+  });
+
+  describe('when the enum is a numeric enum', () => {
+    enum Choice {
+      carrot = 0,
+      stick = 1
+    }
+
+    it('decodes the enum', () => {
+      expect(enumerator(Choice)(0)).toEqual(Choice.carrot);
+    });
+
+    it('throws when the value is not a member of the enum', () => {
+      expect(() => enumerator(Choice)('banana')).toThrowError(DecoderError);
+    });
+  });
+
+
+  describe('when the enum has no values assigned', () => {
+    enum Choice {
+      carrot ,
+      stick
+    }
+
+    it('decodes the enum using the value', () => {
+      expect(enumerator(Choice)(1)).toEqual(Choice.stick);
+    });
+
+    it('throws when the value is not a member of the enum', () => {
+      expect(() => enumerator(Choice)(3)).toThrowError(DecoderError);
+    });
+  });
+})
+
 describe('decoding a complex object', () => {
   type Blob = {
     name: string;

src/lib/json-decode.ts

@@ -114,3 +114,16 @@ export const nullable =
   <T>(decoder: Decoder<T>): Decoder<T | null> =>
   (json) =>
     json === null ? json : decoder(json);
+
+export const enumerator = <T>(enumType: T) => (json: unknown): T[keyof T] => {
+  const entries = Object.entries(enumType as any);
+  const entry = entries.find(([, v]) => v === json);
+  if (entry === undefined) {
+    throw new DecoderError(`Expected enum value, got ${JSON.stringify(json)}`);
+  }
+  const value = entry[1];
+  if (value === undefined) {
+    throw new DecoderError(`Expected enum value, got ${JSON.stringify(json)}`);
+  }
+  return value as T[keyof T];
+}