Add doc

docs/docs/04_client.md

@@ -42,14 +42,15 @@ const data = await res.json(); // data is { userNames: string[] }
 If the response have multiple status codes, response type is union of each status code type.
 
 ```typescript
+type Headers = { headers: { 'Content-Type': 'application/json' } };
 type Spec = DefineApiEndpoints<{
   "/users": {
     get: {
       responses: {
-        200: { body: { names: string[] }; };
-        201: { body: { ok: boolean }; };
-        400: { body: { message: string; }; };
-        500: { body: { internalError: string; }; };
+        200: { body: { names: string[] }; } & Headers;
+        201: { body: { ok: boolean }; } & Headers;
+        400: { body: { message: string; }; } & Headers;
+        500: { body: { internalError: string; }; } & Headers;
       };
     };
   }
@@ -61,6 +62,12 @@ if (!res.ok) {
   // If res.ok is false, status code is 400 or 500
   // So res.json() returns { message: string } | { internalError: string }
   const data = await res.json();
+
+  // Response headers are also type-checked. Content-Type is always 'application/json'
+  const contentType: 'application/json' = res.headers.get('Content-Type');
+  // and, hasContentType is inferred as true, not boolean
+  const hasContentType: true = res.headers.has('Content-Type');
+
   return console.error(data);
 }
 // If res.ok is true, status code is 200 or 201
@@ -69,6 +76,20 @@ const data = await res.json(); // names is string[]
 console.log(data);
 ```
 
+:::info[Response headers limitation]
+
+Response headers are treated as an immutable object for strict type checking.
+It means that you can not `append`, `set` or `delete` operation after the response object is created.
+This is a limitation of the type system, not a runtime change. If you need mutable operations, you can cast types.
+
+```typescript
+const immutableHeaders = res.headers
+const mutableHeaders = res.headers as Headers;
+```
+
+:::
+
+
 ### Path & Path parameters
 
 zero-fetch accepts only the path that is defined in the API specification.

src/fetch/index.t-test.ts

@@ -36,7 +36,12 @@ type ValidateUrlTestCase = [
   type Spec = DefineApiEndpoints<{
     "/users": {
       get: {
-        responses: { 200: { body: { prop: string } } };
+        responses: {
+          200: {
+            body: { prop: string };
+            headers: { "Content-Type": "application/json" };
+          };
+        };
       };
     };
   }>;
@@ -49,6 +54,11 @@ type ValidateUrlTestCase = [
       // methodを省略した場合はgetとして扱う
       const res = await f("/users", {});
       (await res.json()).prop;
+
+      // eslint-disable-next-line @typescript-eslint/no-unused-vars
+      const contentType: "application/json" = res.headers.get("Content-Type");
+      // eslint-disable-next-line @typescript-eslint/no-unused-vars
+      const hasContentType: true = res.headers.has("Content-Type");
     }
   })();
 }