docs: improve API mocking docs (#19189)

docs/src/api/class-route.md

@@ -430,7 +430,8 @@ await page.route('https://dog.ceo/api/breeds/list/all', async route => {
 page.route("https://dog.ceo/api/breeds/list/all", route -> {
   APIResponse response = route.fetch();
   JsonObject json = new Gson().fromJson(response.text(), JsonObject.class);
-  json.set("big_red_dog", new JsonArray());
+  JsonObject message = itemObj.get("json").getAsJsonObject();
+  message.set("big_red_dog", new JsonArray());
   route.fulfill(new Route.FulfillOptions()
     .setResponse(response)
     .setBody(json.toString()));
@@ -441,7 +442,7 @@ page.route("https://dog.ceo/api/breeds/list/all", route -> {
 async def handle(route):
     response = await route.fulfill()
     json = await response.json()
-    json["big_red_dog"] = []
+    json["message"]["big_red_dog"] = []
     await route.fulfill(response=response, json=json)
 
 await page.route("https://dog.ceo/api/breeds/list/all", handle)
@@ -451,7 +452,7 @@ await page.route("https://dog.ceo/api/breeds/list/all", handle)
 def handle(route):
     response = route.fulfill()
     json = response.json()
-    json["big_red_dog"] = []
+    json["message"]["big_red_dog"] = []
     route.fulfill(response=response, json=json)
 
 page.route("https://dog.ceo/api/breeds/list/all", handle)
@@ -462,7 +463,7 @@ await page.RouteAsync("https://dog.ceo/api/breeds/list/all", async route =>
 {
     var response = await route.FetchAsync();
     dynamic json = await response.JsonAsync();
-    json.big_red_dog = new string[] {};
+    json.message.big_red_dog = new string[] {};
     await route.FulfillAsync(new() { Response = response, Json = json });
 });
 ```

docs/src/mock-browser-js.md

@@ -1,6 +1,6 @@
 ---
-id: mock
-title: "Mock APIs"
+id: mock-browser-apis
+title: "Mock Browser APIs"
 ---
 
 Playwright provides native support for most of the browser features. However, there are some experimental APIs

docs/src/mock.md

@@ -0,0 +1,123 @@
+---
+id: mocj
+title: "Mock APIs"
+---
+
+Web APIs are usually implemented as HTTP endpoints. Playwright provides APIs to **mock** and **modify** network traffic, both HTTP and HTTPS. Any requests that a page does, including [XHRs](https://developer.mozilla.org/en-US/docs/Web/API/XMLHttpRequest) and
+[fetch](https://developer.mozilla.org/en-US/docs/Web/API/Fetch_API) requests, can be tracked, modified and mocked.
+
+## Mock API requests
+
+Following code will intercept all the calls to `https://dog.ceo/api/breeds/list/all` and will return
+the test data instead. No requests to the `https://dog.ceo/api/breeds/list/all` endpoint will be made.
+
+Read more about [advanced networking](./network.md).
+
+```js
+await page.route('https://dog.ceo/api/breeds/list/all', async route => {
+  const json = {
+    message: { 'test_breed': [] }
+  };
+  await route.fulfill({ json });
+});
+```
+
+```python async
+async def handle(route):
+    json = { message: { "test_breed": [] } }
+    await route.fulfill(json=json)
+
+await page.route("https://dog.ceo/api/breeds/list/all", handle)
+```
+
+```python sync
+async def handle(route):
+    json = { message: { "test_breed": [] } }
+    route.fulfill(json=json)
+
+page.route("https://dog.ceo/api/breeds/list/all", handle)
+```
+
+```csharp
+await page.RouteAsync("https://dog.ceo/api/breeds/list/all", async route =>
+{
+    var json = /* JsonElement with the test payload */;
+    await route.FulfillAsync(new () { Json: json });
+});
+```
+
+```java
+page.route("https://dog.ceo/api/breeds/list/all", route -> {
+  route.fulfill(new Route.FulfillOptions()
+    .setBody("{\"message\":{\"test_breed\":[]}}"));
+});
+```
+
+## Modify API responses
+
+Sometimes, it is essential to make an API request, but response needs to be patched to
+allow for reproducible testing. In that case, instead of mocking the request, one
+can perform the request and fulfill it with the modified response.
+
+Read more about [advanced networking](./network.md).
+
+```js
+await page.route('https://dog.ceo/api/breeds/list/all', async route => {
+  const response = await route.fetch();
+  const json = await response.json();
+  json.message['big_red_dog'] = [];
+  // Fullfill using the original response, while patching the response body
+  // with the given JSON object.
+  await route.fulfill({ response, json });
+});
+```
+
+```python async
+async def handle(route):
+    response = await route.fulfill()
+    json = await response.json()
+    json["message"]["big_red_dog"] = []
+    # Fullfill using the original response, while patching the response body
+    # with the given JSON object.
+    await route.fulfill(response=response, json=json)
+
+await page.route("https://dog.ceo/api/breeds/list/all", handle)
+```
+
+```python sync
+def handle(route):
+    response = route.fulfill()
+    json = response.json()
+    json["message"]["big_red_dog"] = []
+    # Fullfill using the original response, while patching the response body
+    # with the given JSON object.
+    route.fulfill(response=response, json=json)
+
+page.route("https://dog.ceo/api/breeds/list/all", handle)
+```
+
+```csharp
+await page.RouteAsync("https://dog.ceo/api/breeds/list/all", async route =>
+{
+    var response = await route.FetchAsync();
+    dynamic json = await response.JsonAsync();
+    json.message.big_red_dog = new string[] {};
+    // Fullfill using the original response, while patching the response body
+    // with the given JSON object.
+    await route.FulfillAsync(new() { Response = response, Json = json });
+});
+```
+
+```java
+page.route("https://dog.ceo/api/breeds/list/all", route -> {
+  APIResponse response = route.fetch();
+  JsonObject json = new Gson().fromJson(response.text(), JsonObject.class);
+  JsonObject message = json.get("message").getAsJsonObject();
+  message.set("big_red_dog", new JsonArray());
+  // Fullfill using the original response, while patching the response body
+  // with the given JSON object.
+  route.fulfill(new Route.FulfillOptions()
+    .setResponse(response)
+    .setBody(json.toString()));
+});
+```