feat: infer health check result

lib/health-check/health-check-executor.service.ts

@@ -5,6 +5,7 @@ import {
 } from './health-check-result.interface';
 import { type HealthCheckError } from '../health-check/health-check.error';
 import {
+  type InferHealthIndicatorResults,
   type HealthIndicatorFunction,
   type HealthIndicatorResult,
 } from '../health-indicator';
@@ -37,13 +38,15 @@ export class HealthCheckExecutor implements BeforeApplicationShutdown {
    * @returns the result of given health indicators
    * @param healthIndicators The health indicators which should get executed
    */
-  async execute(
-    healthIndicators: HealthIndicatorFunction[],
-  ): Promise<HealthCheckResult> {
+  async execute<const TFns extends HealthIndicatorFunction[]>(
+    healthIndicators: TFns,
+  ) {
     const { results, errors } =
       await this.executeHealthIndicators(healthIndicators);
 
-    return this.getResult(results, errors);
+    return this.getResult(results, errors) as HealthCheckResult<
+      InferHealthIndicatorResults<TFns>
+    >;
   }
 
   /**

lib/health-check/health-check-result.interface.ts

@@ -9,7 +9,15 @@ export type HealthCheckStatus = 'error' | 'ok' | 'shutting_down';
  * The result of a health check
  * @publicApi
  */
-export interface HealthCheckResult {
+export type HealthCheckResult<
+  TDetails extends HealthIndicatorResult = HealthIndicatorResult,
+  TInfo extends Partial<HealthIndicatorResult> | undefined =
+    | Partial<TDetails>
+    | undefined,
+  TError extends Partial<HealthIndicatorResult> | undefined =
+    | Partial<TDetails>
+    | undefined,
+> = {
   /**
    * The overall status of the Health Check
    */
@@ -18,14 +26,14 @@ export interface HealthCheckResult {
    * The info object contains information of each health indicator
    * which is of status "up"
    */
-  info?: HealthIndicatorResult;
+  info?: TInfo;
   /**
    * The error object contains information of each health indicator
    * which is of status "down"
    */
-  error?: HealthIndicatorResult;
+  error?: TError;
   /**
    * The details object contains information of every health indicator.
    */
-  details: HealthIndicatorResult;
-}
+  details: TDetails;
+};

lib/health-check/health-check.service.ts

@@ -4,11 +4,11 @@ import {
   Inject,
   ConsoleLogger,
   LoggerService,
+  InternalServerErrorException,
 } from '@nestjs/common';
 import { ErrorLogger } from './error-logger/error-logger.interface';
 import { ERROR_LOGGER } from './error-logger/error-logger.provider';
 import { HealthCheckExecutor } from './health-check-executor.service';
-import { type HealthCheckResult } from './health-check-result.interface';
 import { type HealthIndicatorFunction } from '../health-indicator';
 import { TERMINUS_LOGGER } from './logger/logger.provider';
 
@@ -43,22 +43,31 @@ export class HealthCheckService {
    * ```
    * @param healthIndicators The health indicators which should be checked
    */
-  async check(
-    healthIndicators: HealthIndicatorFunction[],
-  ): Promise<HealthCheckResult> {
+  async check<const TFns extends HealthIndicatorFunction[]>(
+    healthIndicators: TFns,
+  ) {
     const result = await this.healthCheckExecutor.execute(healthIndicators);
-    if (result.status === 'ok') {
-      return result;
-    }
 
-    if (result.status === 'error') {
-      const msg = this.errorLogger.getErrorMessage(
-        'Health Check has failed!',
-        result.details,
-      );
-      this.logger.error(msg);
-    }
+    switch (result.status) {
+      case 'ok':
+        return result;
 
-    throw new ServiceUnavailableException(result);
+      case 'error':
+        const msg = this.errorLogger.getErrorMessage(
+          'Health Check has failed!',
+          result.details,
+        );
+        this.logger.error(msg);
+        throw new ServiceUnavailableException(result);
+
+      case 'shutting_down':
+        throw new ServiceUnavailableException(result);
+
+      default:
+        // Ensure that we have exhaustively checked all cases
+        // eslint-disable-next-line unused-imports/no-unused-vars
+        const exhaustiveCheck: never = result.status;
+        throw new InternalServerErrorException();
+    }
   }
 }

lib/health-indicator/health-indicator-result.interface.ts

@@ -1,3 +1,5 @@
+import { type HealthIndicatorFunction } from './health-indicator';
+
 /**
  * @publicApi
  */
@@ -12,3 +14,24 @@ export type HealthIndicatorResult<
   Status extends HealthIndicatorStatus = HealthIndicatorStatus,
   OptionalData extends Record<string, any> = Record<string, any>,
 > = Record<Key, { status: Status } & OptionalData>;
+
+/**
+ * @internal
+ */
+export type InferHealthIndicatorResult<
+  Fn extends HealthIndicatorFunction = HealthIndicatorFunction,
+> = Awaited<ReturnType<Fn>>;
+
+/**
+ * @internal
+ */
+export type InferHealthIndicatorResults<
+  Fns extends readonly HealthIndicatorFunction[] = HealthIndicatorFunction[],
+  R extends readonly any[] = [],
+> = Fns extends readonly [
+  infer Fn extends HealthIndicatorFunction,
+  ...infer Rest extends readonly HealthIndicatorFunction[],
+]
+  ? InferHealthIndicatorResults<Rest, [...R, Fn]> &
+      InferHealthIndicatorResult<Fn>
+  : HealthIndicatorResult;

lib/health-indicator/health-indicator.service.ts

@@ -28,13 +28,13 @@ export class HealthIndicatorSession<Key extends Readonly<string> = string> {
    */
   down<T extends AdditionalData>(
     data?: T,
-  ): HealthIndicatorResult<typeof this.key, 'down', T>;
+  ): HealthIndicatorResult<Key, 'down', T>;
   down<T extends string>(
     data?: T,
-  ): HealthIndicatorResult<typeof this.key, 'down', { message: T }>;
+  ): HealthIndicatorResult<Key, 'down', { message: T }>;
   down<T extends AdditionalData | string>(
     data?: T,
-  ): HealthIndicatorResult<typeof this.key, 'down'> {
+  ): HealthIndicatorResult<Key, 'down'> {
     let additionalData: AdditionalData = {};
 
     if (typeof data === 'string') {