Allow default behavior with HTTP interceptors

README.md

@@ -222,7 +222,15 @@ The `InMemoryDbService` method name must be the same as the HTTP method name but
 This service calls it with an `HttpMethodInterceptorArgs` object.
 For example, your HTTP GET interceptor would be called like this:
 e.g., `yourInMemDbService["get"](interceptorArgs)`.
-Your method must **return an `Observable<Response>`** which _should be "cold"_.
+
+Your method can have two possible returns:
+* `null` - the service will continue on with its default processing (checking the collection or
+falling back to a different backend).  This is useful if you only wish to handle processing
+for certain paths in the given HTTP method.
+* `Observable<Response>` - your code has handled the request and the response is available from this
+observable.  It _should be "cold"_.
+
+Your method must **return one of these two values**.
 
 The `HttpMethodInterceptorArgs` (as of this writing) are:
 ```ts

src/in-memory-backend.service.ts

@@ -343,7 +343,10 @@ export class InMemoryBackendService {
     if (/commands\/$/i.test(reqInfo.base)) {
       return this.commands(reqInfo);
 
-    } else if (this.inMemDbService[reqMethodName]) {
+    }
+
+    // Don't do an else if to allow interceptor to fall back to default behavior below.
+    if (this.inMemDbService[reqMethodName]) {
       // InMemoryDbService has an overriding interceptor for this HTTP method; call it
       // The interceptor result must be an Observable<Response>
       const interceptorArgs: HttpMethodInterceptorArgs = {
@@ -352,10 +355,15 @@ export class InMemoryBackendService {
         config: this.config,
         passThruBackend: this.passThruBackend
       };
-      const interceptorResponse = this.inMemDbService[reqMethodName](interceptorArgs) as Observable<Response>;
-      return this.addDelay(interceptorResponse);
 
-    } else if (reqInfo.collection) {
+      const interceptorResponse = this.inMemDbService[reqMethodName](interceptorArgs);
+      if (interceptorResponse !== null) {
+        return this.addDelay(interceptorResponse as Observable<Response>);
+      }
+    }
+
+    // Standard behavior without interceptors
+    if (reqInfo.collection) {
       // request is for a collection created by the InMemoryDbService
       return this.addDelay(this.collectionHandler(reqInfo));