Add node:test runner and unit tests for API exports

.nvmrc

@@ -0,0 +1 @@
+lts/*

package.json

@@ -107,6 +107,7 @@
     "test": "npm-run-all build -p -r test:*",
     "test:e2e": "wdio wdio.conf.cjs",
     "test:server": "node test/server.js",
+    "test:unit": "node --test test/unit/",
     "start": "run-s build:ts test:server watch",
     "watch": "run-p watch:*",
     "watch:ts": "tsc -b -w",

test/unit/index-test.js

@@ -0,0 +1,33 @@
+import {describe, it} from 'node:test';
+import assert from 'assert';
+import {
+  onCLS,
+  onFCP,
+  onFID,
+  onINP,
+  onLCP,
+  onTTFB,
+  CLSThresholds,
+  FCPThresholds,
+  FIDThresholds,
+  INPThresholds,
+  LCPThresholds,
+  TTFBThresholds,
+} from 'web-vitals';
+
+describe('index', () => {
+  it('exports Web Vitals metrics functions', () => {
+    [onCLS, onFCP, onFID, onINP, onLCP, onTTFB].forEach((onFn) =>
+      assert(typeof onFn === 'function')
+    );
+  });
+
+  it('exports Web Vitals metric thresholds', () => {
+    assert.deepEqual(CLSThresholds, [0.1, 0.25]);
+    assert.deepEqual(FCPThresholds, [1800, 3000]);
+    assert.deepEqual(FIDThresholds, [100, 300]);
+    assert.deepEqual(INPThresholds, [200, 500]);
+    assert.deepEqual(LCPThresholds, [2500, 4000]);
+    assert.deepEqual(TTFBThresholds, [800, 1800]);
+  });
+});