Add ability to specify jsonrpc context as extra argument

In this case the interface does not define context, but the implementing
service may use it as an optional argument:

  interface IService {
    add(a: number, b: number): number
  }

  class Service implements IService {
    add(a: number, b: number, ctx?: IContext): number {
      return a + b + ctx!.userId
    }
  }

packages/jsonrpc/src/express.test.ts

@@ -19,6 +19,7 @@ describe('jsonrpc', () => {
     httpError(statusCode: number, message: string): Promise<void>
 
     addWithContext(a: number, b: number): (ctx: IContext) => number
+    addWithContext2(a: number, b: number): Promise<number>
   }
 
   class Service implements IService {
@@ -51,6 +52,9 @@ describe('jsonrpc', () => {
     addWithContext = (a: number, b: number) => (ctx: IContext): number => {
       return a + b + ctx.userId
     }
+    addWithContext2(a: number, b: number, ctx?: IContext) {
+      return Promise.resolve(a + b + ctx!.userId)
+    }
   }
 
   function createApp() {
@@ -65,6 +69,7 @@ describe('jsonrpc', () => {
         'asyncError',
         'httpError',
         'addWithContext',
+        'addWithContext2',
       ])
       .router(),
     )
@@ -144,6 +149,10 @@ describe('jsonrpc', () => {
       const response = await client.addWithContext(5, 7)
       expect(response).toEqual(1000 + 5 + 7)
     })
+    it('can use context as extra argument', async () => {
+      const response = await client.addWithContext2(5, 7)
+      expect(response).toEqual(1000 + 5 + 7)
+    })
     it('handles synchronous notifications', async () => {
       await request(createApp())
       .post('/myService')

packages/jsonrpc/src/jsonrpc.ts

@@ -107,7 +107,7 @@ export const createRpcService = <T, M extends FunctionPropertyNames<T>>(
         })
       }
 
-      let retValue = (rpcService[method] as any)(...params)
+      let retValue = (rpcService[method] as any)(...params, context)
 
       if (typeof retValue === 'function') {
         retValue = retValue(context)