Add pure state-merge module with unit tests

desktop-client/state.js

@@ -0,0 +1,23 @@
+const initialState = Object.freeze({
+  title: '',
+  artist: '',
+  album: '',
+  albumArt: '',
+  isPlaying: false,
+  progress: 0,
+  duration: 0,
+});
+
+function mergeState(current, update) {
+  return {
+    title: update.title ?? current.title,
+    artist: update.artist ?? current.artist,
+    album: update.album ?? current.album,
+    albumArt: update.albumArt ?? current.albumArt,
+    isPlaying: update.isPlaying ?? current.isPlaying,
+    progress: update.progress ?? current.progress,
+    duration: update.duration ?? current.duration,
+  };
+}
+
+module.exports = { initialState, mergeState };

desktop-client/tests/state.test.js

@@ -0,0 +1,47 @@
+const test = require('node:test');
+const assert = require('node:assert/strict');
+const { initialState, mergeState } = require('../state');
+
+test('initialState has the expected shape', () => {
+  assert.deepEqual(initialState, {
+    title: '',
+    artist: '',
+    album: '',
+    albumArt: '',
+    isPlaying: false,
+    progress: 0,
+    duration: 0,
+  });
+});
+
+test('mergeState applies a single field update', () => {
+  const next = mergeState(initialState, { title: 'Foo' });
+  assert.equal(next.title, 'Foo');
+});
+
+test('mergeState preserves fields not present in the update', () => {
+  const current = { ...initialState, title: 'Old', artist: 'Bar' };
+  const next = mergeState(current, { title: 'New' });
+  assert.equal(next.artist, 'Bar');
+});
+
+test('mergeState treats undefined fields as no-change', () => {
+  const current = { ...initialState, isPlaying: true };
+  const next = mergeState(current, { isPlaying: undefined });
+  assert.equal(next.isPlaying, true);
+});
+
+test('mergeState applies false as a real value (not a no-change)', () => {
+  const current = { ...initialState, isPlaying: true };
+  const next = mergeState(current, { isPlaying: false });
+  assert.equal(next.isPlaying, false);
+});
+
+test('mergeState does not mutate inputs', () => {
+  const current = { ...initialState };
+  const update = { title: 'X' };
+  const before = { ...current };
+  mergeState(current, update);
+  assert.deepEqual(current, before);
+  assert.deepEqual(update, { title: 'X' });
+});