josephdpurcell · September 7, 2022 14:14 · Sep 7, 2022
diff --git a/my-decorator.ts b/my-decorator.ts
@@ -0,0 +1,25 @@
+import { ArgumentMetadata, createParamDecorator, ExecutionContext } from '@nestjs/common';
+import { METADATA__PARAM_TYPE } from '../constants'; // METADATA__PARAM_TYPE = 'myType'
+import { paramTypeEnhancer } from './param-type.enhancer';
+
+export const MyDecorator = createParamDecorator(
+  async (data: unknown, ctx: ExecutionContext): Promise<any> => {
+    const metatype = Reflect.getOwnMetadata(METADATA__PARAM_TYPE, ctx.getHandler());
+
+    // We build the argument metadata that @nestjs/class-validator is expecting.
+    // Specifically, we specify the type as "custom" -- this is a special value
+    // coming from NestJS and all custom decorators are "custom". The data is
+    // left undefined (only a @Query or @Param type would have data). Last, the
+    // metatype is the reference to the type.
+    const argument: ArgumentMetadata = {
+      type: 'custom',
+      data: undefined,
+      metatype: metatype,
+    };
+
+    // Do processing here. You can even return a promise.
+    // Often you want access to the request which can be done like this:
+    const request = ctx.switchToHttp().getRequest();
+  },
+  [paramTypeEnhancer],
+);
diff --git a/param-type.enhancer.ts b/param-type.enhancer.ts
@@ -0,0 +1,22 @@
+import { ParamDecoratorEnhancer } from '@nestjs/common';
+import { METADATA__PARAM_TYPE } from '../constants'; // METADATA__PARAM_TYPE = 'myType'
+
+/**
+ * This enhancer grabs the Typescript type of the parameter and shoves it into
+ * the metadata. We do this because the CustomParamFactory does not have access
+ * to the type of param.
+ */
+export const paramTypeEnhancer: ParamDecoratorEnhancer = (
+  target: Record<string, unknown>,
+  propertyKey: string,
+  parameterIndex: number,
+): void => {
+  // Typescript adds the "design:paramtypes" metadata with an array of class
+  // types where the keys are the method argument index and the value is the
+  // class type.
+  const paramTypes = Reflect.getOwnMetadata('design:paramtypes', target, propertyKey);
+  // We can use the parameterIndex to retrieve the specific type we want.
+  const metatype = paramTypes[parameterIndex];
+  // Now, we assign the parameter type to the metadata at a key we know.
+  Reflect.defineMetadata(METADATA__PARAM_TYPE, metatype, target[propertyKey]);
+};