chrome-extension-messaging-tests

Overview

Extension components run in separate contexts (content script, service worker, popup, options page, an external web page, a native host process) and can only reach each other through the messaging APIs described on the Chrome Extensions message passing page (opens in new window). That page defines five distinct shapes, each with its own failure mode:

1. One-shot runtime messages inside one extension (chrome.runtime.sendMessage).
2. One-shot tabs messages into a content script (chrome.tabs.sendMessage).
3. Long-lived ports (chrome.runtime.connect).
4. Cross-extension and web-page messages, gated by externally_connectable.
5. Native messaging (chrome.runtime.connectNative).

Most "the feature silently does nothing" bugs in an extension are one of these five failing quietly: a channel closed before the response, a message sent to a tab with no matching content script, a port dropped by a suspended service worker, or an origin missing from an allow-list. Tests here make each of those loud.

Scope boundary

This skill owns assertions about messages once an extension is installed and running: who can send, who receives, what the response contract is, when a channel closes, and what a payload is allowed to contain.

It deliberately does not cover getting the extension installed, reloaded, or refreshed after a code edit. Those are a separate job with a separate symptom: an install problem shows up as an extension that is absent or shows an error before any test runs, while everything here shows up as a running extension whose messages go nowhere. Assume the extension under test is loaded and its ID is known before applying anything below.

When to use

• sendMessage resolves undefined or the callback never fires, and you need a test that pins down which side dropped the channel.
• A popup action does nothing on the page: the message may never be reaching the content script.
• A long-lived connection dies partway through a session and you need to distinguish a deliberate disconnect() from an unloaded frame.
• A companion web site must talk to the extension and the externally_connectable allow-list has to be proven correct before publishing.
• A native helper application is part of the product and CI has to prove the host is reachable.

How to use

1. Identify the shape. Map the failing (or new) path to one of the five messaging shapes in the Overview - one-shot runtime, one-shot tabs, long-lived port, external / web-page, or native.
2. Confirm the extension is running. Install / reload is out of scope (see Scope boundary); assume the extension is loaded and its ID is known before writing an assertion.
3. Write the sender / listener pair for that shape. The two one-shot forms are in Authoring below; ports and the cross-boundary surfaces are in the linked references.
4. Assert the full response object with an equality matcher, never toBeDefined() - the failure this API produces most often is an undefined-shaped response that toBeDefined() passes on.
5. Apply the payload guardrails. Build fixtures from JSON-representable values only, and route on a message-type field so exactly one onMessage listener owns each type.
6. Re-check the draft against the Anti-patterns table before committing.

Authoring

1. One-shot runtime messages

Per cr-msg (opens in new window), the sender awaits a promise and the listener responds:

// Sender (content script, popup, or options page)
const response = await chrome.runtime.sendMessage({ greeting: 'hello' });

// Listener (service worker)
chrome.runtime.onMessage.addListener((message, sender, sendResponse) => {
  if (message.greeting !== 'hello') return;
  fetch('https://example.com')
    .then(r => sendResponse({ statusCode: r.status }));
  return true;  // hold the channel open for the async sendResponse
});

The return true is load-bearing. Per cr-msg (opens in new window), to respond asynchronously using sendResponse() you must return "a literal true (not just a truthy value)". Returning a truthy non-literal, or returning nothing, closes the channel and the sender's promise settles without the response.

From Chrome 148 a listener may instead return a promise whose resolved value becomes the response, per cr-msg (opens in new window). The same page notes promise support "is not enabled in extension script contexts if the extension extends DevTools with a devtools page", so a DevTools-extending extension must keep using the return true form.

The async-listener trap. An async listener implicitly returns a promise, which is not the literal true that cr-msg (opens in new window) requires. On a Chrome older than 148, or in a DevTools-extending extension, that promise is ignored and the channel closes. Author either a non-async listener with return true, or an async listener that returns the response value.

Test to write:

test('status request resolves with the fetched status code', async () => {
  const response = await sendFromContentScript({ greeting: 'hello' });
  expect(response).toEqual({ statusCode: 200 });   // not toBeDefined()
});

Assert the full response object. A toBeDefined() assertion passes on the undefined-shaped failure this API produces most often.

2. One-shot tabs messages

Per cr-msg (opens in new window), drive a specific tab's content script from the popup or service worker:

chrome.tabs.sendMessage(tab.id, { greeting: 'hello' }, response => {
  const resp = JSON.parse(response.farewell);
});

The chrome.tabs reference (opens in new window) gives the signature as chrome.tabs.sendMessage(tabId, message, options?) returning a promise, with options.frameId to target one frame instead of all frames and options.documentId (Chrome 106 and later) to target one document. Per cr-tabs (opens in new window), "If an error occurs while connecting to the specified tab, the promise will be rejected" - which is the assertion to write for the no-content-script-injected case.

Test targets:

• Happy path: a tab with the content script injected resolves the expected response body.
• Rejection path: a tab whose URL does not match any content_scripts pattern rejects the promise. Assert the rejection, not a timeout.
• Frame targeting: with an iframe present, frameId limits delivery to that frame per cr-tabs (opens in new window).

Long-lived ports, external, and native surfaces

The three remaining shapes carry more incidental depth - port lifecycle tied to the service worker, the externally_connectable allow-list, and machine-level native-host registration - so their code recipes live in two companion references:

• Long-lived ports + service-worker lifecycle - connect / onConnect, the onDisconnect conditions, the wrong-port-name disconnect test, and the 30-second idle-termination interaction: references/long-lived-ports.md.
• Cross-boundary messages (web-page, cross-extension, native) - externally_connectable defaults, onMessageExternal, connectNative, and the per-OS native-host manifest table: references/cross-boundary-messaging.md.

Payload constraints every test must respect

Per cr-msg (opens in new window), the maximum size of a message is 64 MiB, and Chrome uses JSON serialization, "different to other browsers which implement the same APIs with the structured clone algorithm."

That single sentence invalidates a class of test fixtures. Under JSON serialization:

Build fixtures out of plain JSON-representable objects, and assert on the serialized shape the receiver actually observes rather than on identity with the object you sent. A cross-browser suite should note that the same fixture behaves differently elsewhere, since cr-msg (opens in new window) attributes structured clone to other browsers implementing these APIs.

Per cr-msg (opens in new window), when several onMessage listeners are registered, "Only the first listener to respond, reject, or throw an error will affect the sender; all other listeners will run, but their results will be ignored." Any test asserting a specific response in a codebase with more than one listener is implicitly asserting listener order, which is fragile. Prefer routing on a message-type field so exactly one listener can respond to a given message.

Errors surface in two shapes per cr-runtime (opens in new window): callback-style calls set chrome.runtime.lastError, described as "Populated with an error message if calling an API function fails; otherwise undefined", while "API functions that return promises do not set this property" and reject instead. A test must check the mechanism matching the call style it used, or a real failure reads as a pass.

Worked example: a full messaging regression suite

runtime one-shot
  [x] resolves the full response object for a known message type
  [x] async listener without `return true` is caught (response is undefined)
  [x] unknown message type produces no response

tabs
  [x] matching tab resolves the response body
  [x] non-matching tab rejects the promise
  [x] frameId targets one frame only

ports
  [x] named port receives the expected reply sequence
  [x] wrong port name yields exactly one onDisconnect and no reply
  [x] explicit disconnect() records exactly one onDisconnect
  [x] traffic every <30s keeps the port alive across the idle window

external
  [x] allow-listed origin gets a response
  [x] non-allow-listed origin gets none
  [x] non-allow-listed sender extension id is ignored
  [x] adding `matches` did not break extension-to-extension sends

payload
  [x] Date round-trips as an ISO string, not a Date
  [x] Map serializes to {}
  [x] a message over 64 MiB fails rather than silently truncating

Anti-patterns

Limitations

• Cross-extension tests need both extensions present. A chrome.runtime.sendMessage(otherExtensionId, ...) test is meaningless unless the recipient extension is also installed in the same profile.
• Native messaging needs a machine-level install. The host manifest must exist at the per-OS path listed in references/cross-boundary-messaging.md per cr-native (opens in new window), so CI images need an explicit setup step; this cannot be arranged from inside the extension.
• Service-worker lifetime bounds long-running assertions. Per cr-sw (opens in new window) the worker terminates after 30 seconds of inactivity, so "wait a minute then assert" is not a valid test shape.
• externally_connectable is a Chromium manifest key. A cross-browser suite has to branch: the key is declared in the Chromium manifest documentation at cr-extconn (opens in new window), and a browser that does not implement it will not gate connections the same way.
• Behaviour differs by serialization model across browsers. Per cr-msg (opens in new window) Chrome uses JSON serialization while other browsers implementing the same APIs use structured clone, so payload assertions are not portable without branching.

References

• Chrome Extensions, Message passing (all five messaging shapes, return true, onDisconnect conditions, externally_connectable examples, 64 MiB cap, JSON serialization, first-listener-wins, extension-to-page direction): https://developer.chrome.com/docs/extensions/develop/concepts/messaging
• chrome.runtime API reference (connect / connectInfo.name, onMessageExternal, connectNative, lastError versus promise rejection): https://developer.chrome.com/docs/extensions/reference/api/runtime
• chrome.tabs API reference (sendMessage signature, frameId, documentId, rejection on connect failure): https://developer.chrome.com/docs/extensions/reference/api/tabs
• externally_connectable manifest key (defaults, ids, matches): https://developer.chrome.com/docs/extensions/reference/manifest/externally-connectable
• Chrome Extensions, Native messaging (host manifest locations, allowed_origins, 1 MB inbound / 64 MiB outbound limits): https://developer.chrome.com/docs/extensions/develop/concepts/native-messaging
• Extension service worker lifecycle (30-second idle termination, long-lived messaging keeping the worker alive): https://developer.chrome.com/docs/extensions/develop/concepts/service-workers/lifecycle
• Deep reference - long-lived ports + service-worker lifecycle: references/long-lived-ports.md.
• Deep reference - cross-boundary messaging (web-page, cross-extension, native): references/cross-boundary-messaging.md.