test: cover chatPrefillFromUser hook handler

Direct-import test of static/js/index.js's chatPrefillFromUser
export with a stubbed window.clientVars. Five cases:
- AI author + configured trigger -> trigger string returned
- AI author + custom trigger -> custom string returned
- AI author with trigger unset -> falls back to "@ai "
- Non-AI author -> returns undefined so core uses its default
- clientVars.ep_ai_chat missing -> returns undefined (graceful)

The browser-side behavior (chat opens, input is filled, hover style
on the AI row) is covered by the matching Playwright spec in core
PR #7660 — this test fills the unit-level gap so we can verify the
substitution logic without standing up a full browser.

Author: JohnMcLear

static/tests/backend/specs/chat_prefill.ts

@@ -0,0 +1,95 @@
+'use strict';
+
+import {strict as assert} from 'assert';
+
+/**
+ * The chatPrefillFromUser client hook — exercised here as a plain
+ * function (it's a CommonJS export from static/js/index.js) with a
+ * stubbed `window.clientVars`. Lets us cover the AI-vs-human branch
+ * without standing up a browser.
+ *
+ * The companion frontend test in Etherpad core (#7660) covers what
+ * the browser actually does with the returned string.
+ */
+
+const Module = require('module');
+
+// The static/js client bundle requires nothing Node-specific, so we
+// can require it directly. But it reads `window.clientVars`; install
+// a global shim before requiring.
+const installWindowShim = (clientVars: any) => {
+  // @ts-ignore
+  global.window = {clientVars};
+};
+
+const clearWindowShim = () => {
+  // @ts-ignore
+  delete global.window;
+  // Drop the client module from the require cache so a follow-up
+  // require() re-reads it under the new shim.
+  const path = require.resolve('../../../../static/js/index');
+  delete Module._cache[path];
+};
+
+describe('ep_ai_chat - chatPrefillFromUser hook handler', function () {
+  afterEach(() => clearWindowShim());
+
+  it('returns the configured trigger when the clicked user is the AI', function (done) {
+    installWindowShim({ep_ai_chat: {trigger: '@ai', authorId: 'a.ai_42'}});
+    const {chatPrefillFromUser} = require('../../../../static/js/index');
+    chatPrefillFromUser('chatPrefillFromUser', {
+      authorId: 'a.ai_42',
+      name: 'AI Assistant',
+      prefill: '@AI_Assistant ',
+    }, (out: any) => {
+      assert.equal(out, '@ai ');
+      done();
+    });
+  });
+
+  it('respects a custom trigger string', function (done) {
+    installWindowShim({ep_ai_chat: {trigger: '@bot', authorId: 'a.ai_99'}});
+    const {chatPrefillFromUser} = require('../../../../static/js/index');
+    chatPrefillFromUser('chatPrefillFromUser', {
+      authorId: 'a.ai_99',
+      name: 'AI Assistant',
+      prefill: '@AI_Assistant ',
+    }, (out: any) => {
+      assert.equal(out, '@bot ');
+      done();
+    });
+  });
+
+  it('falls back to default ("@ai ") if trigger is missing', function (done) {
+    installWindowShim({ep_ai_chat: {authorId: 'a.ai_42'}});
+    const {chatPrefillFromUser} = require('../../../../static/js/index');
+    chatPrefillFromUser('chatPrefillFromUser', {
+      authorId: 'a.ai_42', name: 'AI Assistant', prefill: '@AI_Assistant ',
+    }, (out: any) => {
+      assert.equal(out, '@ai ');
+      done();
+    });
+  });
+
+  it('returns nothing for a non-AI author so core uses its default', function (done) {
+    installWindowShim({ep_ai_chat: {trigger: '@ai', authorId: 'a.ai_42'}});
+    const {chatPrefillFromUser} = require('../../../../static/js/index');
+    chatPrefillFromUser('chatPrefillFromUser', {
+      authorId: 'a.alice_1', name: 'Alice', prefill: '@Alice ',
+    }, (out: any) => {
+      assert.equal(out, undefined);
+      done();
+    });
+  });
+
+  it('returns nothing when clientVars.ep_ai_chat is missing', function (done) {
+    installWindowShim({});
+    const {chatPrefillFromUser} = require('../../../../static/js/index');
+    chatPrefillFromUser('chatPrefillFromUser', {
+      authorId: 'a.ai_42', name: 'AI Assistant', prefill: '@AI_Assistant ',
+    }, (out: any) => {
+      assert.equal(out, undefined);
+      done();
+    });
+  });
+});