feat: add isArray.force option for putting singular values into an ar…

…ray of their own

src/core.ts

@@ -23,9 +23,29 @@ export type SingularPropertyOptions<T> = {
 export type ArrayPropertyOptions<T> = {
   example?: T[];
   default?: T[];
-  isArray: true | { minLength?: number; maxLength?: number; length?: number };
+  isArray:
+    | true
+    | {
+        minLength?: number;
+        maxLength?: number;
+        length?: number;
+        /**
+         * Wheter to put singular value into an array of it's own.
+         * Useful if the decorated class is the query DTO.
+         *
+         * *NOTE*: This doesn't create an empty array if the value is not present.
+         */
+        force?: boolean;
+      };
 };
 
+const ForceArrayTransform = Transform(({ value }) => {
+  if (value === undefined) {
+    return value;
+  }
+  return Array.isArray(value) ? value : [value];
+});
+
 export const noop = (): void => undefined;
 
 export const compose = <T, CustomOptions>(
@@ -42,10 +62,11 @@ export const compose = <T, CustomOptions>(
   }: PropertyOptions<T, CustomOptions>,
   ...decorators: PropertyDecorator[]
 ): PropertyDecorator => {
-  const isArrayObj = typeof isArray === 'object' ? isArray : {};
-  const length = isArrayObj.length;
-  const minLength = length ?? isArrayObj.minLength;
-  const maxLength = length ?? isArrayObj.maxLength;
+  const arrayProps = typeof isArray === 'object' ? isArray : {};
+  const length = arrayProps.length;
+  const minLength = length ?? arrayProps.minLength;
+  const maxLength = length ?? arrayProps.maxLength;
+  const forceArray = arrayProps.force ?? false;
 
   return applyDecorators(
     ...decorators,
@@ -55,6 +76,7 @@ export const compose = <T, CustomOptions>(
     !!isArray ? IsArray() : noop,
     minLength ? ArrayMinSize(minLength) : noop,
     maxLength ? ArrayMaxSize(maxLength) : noop,
+    forceArray ? ForceArrayTransform : noop,
     def !== undefined ? Transform(({ value }) => (value === undefined ? def : value)) : noop,
     ApiProperty({
       ...apiPropertyOptions,

src/decorators/is-string.spec.ts

@@ -232,6 +232,61 @@ describe('IsString', () => {
     });
   });
 
+  describe('array (in query)', () => {
+    class Test {
+      @IsString({ optional: true, isArray: { force: true } })
+      stringField?: string[];
+    }
+
+    it('generates correct schema', async () => {
+      expect(await generateSchemas([Test])).toStrictEqual({
+        Test: {
+          type: 'object',
+          properties: {
+            stringField: {
+              type: 'array',
+              items: {
+                type: 'string',
+              },
+            },
+          },
+        },
+      });
+    });
+
+    it('accepts string arrays', async () => {
+      expect(await input(Test, { stringField: ['a', 'b', 'c'] })).toStrictEqual(
+        Result.ok(make(Test, { stringField: ['a', 'b', 'c'] })),
+      );
+      expect(await input(Test, { stringField: [] })).toStrictEqual(
+        Result.ok(make(Test, { stringField: [] })),
+      );
+    });
+
+    it('accepts singular strings and transforms them into arrays', async () => {
+      expect(await input(Test, { stringField: 'a' })).toStrictEqual(
+        Result.ok(make(Test, { stringField: ['a'] })),
+      );
+
+      expect(await input(Test, { stringField: 'bcde' })).toStrictEqual(
+        Result.ok(make(Test, { stringField: ['bcde'] })),
+      );
+    });
+
+    it('works with optionals', async () => {
+      expect(await input(Test, {})).toStrictEqual(Result.ok(make(Test, {})));
+    });
+
+    it('rejects everything else', async () => {
+      expect(await input(Test, { stringField: true })).toStrictEqual(
+        Result.err('each value in stringField must be a string'),
+      );
+      expect(await input(Test, { stringField: [1, 2, 3] })).toStrictEqual(
+        Result.err('each value in stringField must be a string'),
+      );
+    });
+  });
+
   describe('date', () => {
     describe('date', () => {
       class Test {