Transparently inject default values for parameters into handler arguments

packages/cli/package.json

@@ -10,7 +10,7 @@
     "postbuild": "chmod u+x dist/cli.js"
   },
   "dependencies": {
-    "openapi-typescript": "^7.9.1"
+    "openapi-typescript": "https://github.com/luqasn/openapi-typescript/releases/download/v7.11.1-preview/openapi-typescript-7.11.1-preview.tgz"
   },
   "peerDependencies": {
     "typescript": "^5.9.3"

packages/cli/src/typegen/typegen.ts

@@ -17,7 +17,10 @@ function write(dirName: string, fileName: string, data: string) {
 async function createSpecTypes(specPath: string, outputDir: string) {
   const absolutePath = path.resolve(specPath);
   const url = pathToFileURL(absolutePath)
-  const ast = await openapiTS(url);
+
+  const ast = await openapiTS(url, {
+      makeParametersWithDefaultNotUndefined: true
+  });
   const ts = astToString(ast);
 
   return write(outputDir, 'spec.ts', ts);

packages/lib/src/openapi.ts

@@ -137,16 +137,34 @@ export class OpenApi<T> {
     return api;
   }
 
+  protected injectDefaults(parsed: Params, parameterMap: Record<string, OpenAPIV3_1.ParameterBaseObject>): Params {
+    for (const [name, p] of Object.entries(parameterMap)) {
+      if (p.schema === undefined) {
+          continue
+      }
+      if ("$ref" in p.schema) {
+          throw new Error('this should never happen, schema should be dereferenced by openapi-backend')
+      }
+      if (p.schema.default !== undefined && parsed[name] === undefined) {
+          parsed[name] = p.schema.default
+      }
+    }
+    return parsed;
+  }
+
   protected parseParams(rawParams: StringParams,
                         operation: OpenAPI.Operation,
                         type: ParameterType,
                         errors: Ajv.ErrorObject[]): Params {
+    const parameterMap = getParameterMap(operation, type)
     // This is mostly used to coerce types, which openapi-backend does internally but then throws away
-    return matchSchema<StringParams, Params>(
+    const parsed = matchSchema<StringParams, Params>(
         this.paramValidator,
         rawParams,
-        getParametersSchema(getParameterMap(operation, type)),
+        getParametersSchema(parameterMap),
         errors);
+
+    return this.injectDefaults(parsed, parameterMap)
   }
 
   protected parseRequest(apiContext: OpenAPI.Context): Request {

packages/test/api.yml

@@ -20,6 +20,7 @@ components:
   schemas:
     Title:
       type: string
+      default: Mrs
       enum:
         - Mr
         - Mrs

packages/test/src/openapi.test.ts

@@ -11,7 +11,7 @@ function getTypeMap(obj: any) {
 
 const operations: OperationHandlers<unknown> = {
   greet: req => {
-    const {params: {name}, query: {title = ''}} = req;
+    const {params: {name}, query: {title}} = req;
 
     return {
       message: greet(title, name),
@@ -61,6 +61,19 @@ describe('API tests', () => {
       }
     });
     expect(res.statusCode).toEqual(200);
+    expect(res.body).toEqual({message: 'Hello, Mr John Doe'})
+  });
+
+  it('Should set default values for parameters automatically', async () => {
+      const res = await api.handleRequest({
+          method: 'GET',
+          path: '/greet/Jane%20Doe',
+          headers: {
+              authorization: 'true',
+          }
+      });
+      expect(res.statusCode).toEqual(200);
+      expect(res.body).toEqual({message: 'Hello, Mrs Jane Doe'})
   });
 
   it('Should handle a valid request with implicit status 201', async () => {