diff --git a/package.json b/package.json
index 9893263..4a4d278 100644
--- a/package.json
+++ b/package.json
@@ -4,12 +4,13 @@
   "homepage": "https://github.com/iFixit/localize",
   "main": "dist/translations.js",
   "types": "./localize.d.ts",
+  "type": "module",
   "module": "dist/translations.js",
   "files": [
     "dist", "localize.d.ts"
   ],
   "scripts": {
-    "test": "node ./test/test.mjs",
+    "test": "node --test test/",
     "build": "rimraf dist && NODE_ENV=production babel src --out-dir dist --copy-files --ignore __tests__,spec.js,test.js,__snapshots__"
   },
   "devDependencies": {
diff --git a/test/test-uninitialized.mjs b/test/test-uninitialized.mjs
new file mode 100644
index 0000000..65a2aeb
--- /dev/null
+++ b/test/test-uninitialized.mjs
@@ -0,0 +1,24 @@
+import { test, beforeEach } from 'node:test';
+import { strict as assert } from 'node:assert';
+import { _js, setTranslations } from '../src/translations.js';
+
+beforeEach(() => {
+   // Reset the translations state before each test
+   setTranslations(undefined);
+});
+
+test('_js: falls back to returning the input string when no translations are set', (t) => {
+   const errors = [];
+   t.mock.method(console, 'error', (msg) => errors.push(msg));
+   assert.equal(_js('Hello'), 'Hello');
+   assert.equal(errors.length, 1);
+   assert.match(errors[0], /UI Translations not found/);
+});
+
+test('_js: only logs the missing-translations warning once', (t) => {
+   const errors = [];
+   t.mock.method(console, 'error', (msg) => errors.push(msg));
+   _js('first');
+   _js('second');
+   assert.equal(errors.length, 1);
+});
diff --git a/test/test.mjs b/test/test.mjs
index 8869775..62ad3e5 100644
--- a/test/test.mjs
+++ b/test/test.mjs
@@ -1,7 +1,84 @@
-import { _js, setTranslations } from "../dist/translations.js";
-import { strict as assert } from 'assert';
+import { test, beforeEach, afterEach } from 'node:test';
+import { strict as assert } from 'node:assert';
+import { _js, ___p, setTranslations } from '../src/translations.js';
 
-const translatedHi = "Translated Hi (%1)";
+beforeEach(() => {
+   setTranslations({});
+   delete globalThis.window;
+});
 
-setTranslations({Hi: translatedHi});
-assert.equal("Translated Hi (param)", _js("Hi", "param"));
+afterEach(() => {
+   delete globalThis.window;
+});
+
+test('_js: returns the input string when no translation matches', () => {
+   assert.equal(_js('Hello'), 'Hello');
+});
+
+test('_js: returns the translated string when a match is found', () => {
+   setTranslations({ Hello: 'Hola' });
+   assert.equal(_js('Hello'), 'Hola');
+});
+
+test('_js: collapses runs of whitespace before lookup', () => {
+   setTranslations({ 'Foo bar baz': 'translated' });
+   assert.equal(_js('Foo   bar\n\tbaz'), 'translated');
+});
+
+test('_js: returns the compacted string when no match found', () => {
+   assert.equal(_js('Foo   bar'), 'Foo bar');
+});
+
+test('_js: substitutes a single %1 placeholder', () => {
+   setTranslations({ Hi: 'Translated Hi (%1)' });
+   assert.equal(_js('Hi', 'param'), 'Translated Hi (param)');
+});
+
+test('_js: substitutes multiple positional placeholders', () => {
+   assert.equal(_js('a %1 b %2', 'x', 'y'), 'a x b y');
+});
+
+test('_js: replaces every occurrence of a placeholder', () => {
+   assert.equal(_js('%1 and %1', 'foo'), 'foo and foo');
+});
+
+test('_js: coerces non-string input via String()', () => {
+   assert.equal(_js(42), '42');
+});
+
+test('_js: returns "Translated" when window.App.showTranslatedPlaceholder is set', () => {
+   globalThis.window = { App: { showTranslatedPlaceholder: true } };
+   setTranslations({ x: 'translated x' });
+   assert.equal(_js('x'), 'Translated');
+});
+
+test('_js: ignores window.App when the placeholder flag is falsy', () => {
+   globalThis.window = { App: { showTranslatedPlaceholder: false } };
+   setTranslations({ x: 'translated x' });
+   assert.equal(_js('x'), 'translated x');
+});
+
+test('___p: uses the singular form when number === 1', () => {
+   assert.equal(___p(1, '%1 step', '%1 steps', 1), '1 step');
+});
+
+test('___p: uses the plural form when number === 0', () => {
+   assert.equal(___p(0, '%1 step', '%1 steps', 0), '0 steps');
+});
+
+test('___p: uses the plural form when number > 1', () => {
+   assert.equal(___p(5, '%1 step', '%1 steps', 5), '5 steps');
+});
+
+test('___p: forwards args to _js for translation lookup', () => {
+   setTranslations({ '%1 step': 'TS:%1', '%1 steps': 'TP:%1' });
+   assert.equal(___p(1, '%1 step', '%1 steps', 7), 'TS:7');
+   assert.equal(___p(2, '%1 step', '%1 steps', 7), 'TP:7');
+});
+
+test('setTranslations: replaces the translation table on each call', () => {
+   setTranslations({ a: 'first' });
+   assert.equal(_js('a'), 'first');
+   setTranslations({ a: 'second' });
+   assert.equal(_js('a'), 'second');
+});