lib: add navigator.userAgent

PR-URL: nodejs#50200
Reviewed-By: Geoffrey Booth <webadmin@geoffreybooth.com>
Reviewed-By: Matthew Aitken <maitken033380023@gmail.com>
Reviewed-By: Matteo Collina <matteo.collina@gmail.com>
Reviewed-By: Vinícius Lourenço Claro Cardoso <contact@viniciusl.com.br>

doc/api/globals.md

@@ -629,6 +629,21 @@ logical processors available to the current Node.js instance.
 console.log(`This process is running on ${navigator.hardwareConcurrency}`);
 ```
 
+### `navigator.userAgent`
+
+<!-- YAML
+added: REPLACEME
+-->
+
+* {string}
+
+The `navigator.userAgent` read-only property returns user agent
+consisting of the runtime name and major version number.
+
+```js
+console.log(`The user-agent is ${navigator.userAgent}`); // Prints "Node.js/21"
+```
+
 ## `PerformanceEntry`
 
 <!-- YAML

lib/internal/navigator.js

@@ -18,10 +18,12 @@ const {
 } = internalBinding('os');
 
 const kInitialize = Symbol('kInitialize');
+const nodeVersion = process.version;
 
 class Navigator {
   // Private properties are used to avoid brand validations.
   #availableParallelism;
+  #userAgent = `Node.js/${nodeVersion.slice(1, nodeVersion.indexOf('.'))}`;
 
   constructor() {
     if (arguments[0] === kInitialize) {
@@ -37,10 +39,18 @@ class Navigator {
     this.#availableParallelism ??= getAvailableParallelism();
     return this.#availableParallelism;
   }
+
+  /**
+   * @return {string}
+   */
+  get userAgent() {
+    return this.#userAgent;
+  }
 }
 
 ObjectDefineProperties(Navigator.prototype, {
   hardwareConcurrency: kEnumerableProperty,
+  userAgent: kEnumerableProperty,
 });
 
 module.exports = {

test/parallel/test-navigator.js

@@ -13,3 +13,5 @@ const is = {
 is.number(+navigator.hardwareConcurrency, 'hardwareConcurrency');
 is.number(navigator.hardwareConcurrency, 'hardwareConcurrency');
 assert.ok(navigator.hardwareConcurrency > 0);
+assert.strictEqual(typeof navigator.userAgent, 'string');
+assert.match(navigator.userAgent, /^Node\.js\/\d+$/);