lib: add navigator.deviceMemory

Author: Uzlopak

doc/api/globals.md

@@ -652,6 +652,27 @@ consisting of the runtime name and major version number.
 console.log(`The user-agent is ${navigator.userAgent}`); // Prints "Node.js/21"
 ```
 
+### `navigator.deviceMemory`
+
+<!-- YAML
+added: REPLACEME
+-->
+
+> Stability: 1 - Experimental
+
+* {number}
+
+The `navigator.deviceMemory` read-only property indicates the approximate
+amount of available RAM on the device. It is part of the
+[Device Memory API](https://www.w3.org/TR/device-memory/).
+
+The amount of device RAM can be used as a fingerprinting variable, so values
+are intentionally coarse to reduce the potential for its misuse.
+
+```js
+console.log(`The process has approximately ${navigator.deviceMemory} GiB of RAM`);
+```
+
 ## `PerformanceEntry`
 
 <!-- YAML

lib/internal/navigator.js

@@ -1,6 +1,7 @@
 'use strict';
 
 const {
+  NumberParseFloat,
   ObjectDefineProperties,
   Symbol,
 } = primordials;
@@ -15,15 +16,38 @@ const {
 
 const {
   getAvailableParallelism,
+  getTotalMem,
 } = internalBinding('os');
 
 const kInitialize = Symbol('kInitialize');
 const nodeVersion = process.version;
 
+function getApproximatedDeviceMemory(totalMemoryInBytes) {
+  const totalMemoryInMB = totalMemoryInBytes / (1024 * 1024);
+  let mostSignificantByte = 0;
+  let lowerBound = totalMemoryInMB;
+
+  // Extract the most-significant-bit and its location.
+  while (lowerBound > 1) {
+    lowerBound >>= 1;
+    mostSignificantByte++;
+  }
+
+  const upperBound = (lowerBound + 1) << mostSignificantByte;
+  lowerBound = lowerBound << mostSignificantByte;
+
+  // Find the closest bound, and convert it to GB.
+  if (totalMemoryInMB - lowerBound <= upperBound - totalMemoryInMB) {
+    return NumberParseFloat((lowerBound / 1024.0).toFixed(3));
+  }
+  return NumberParseFloat((upperBound / 1024.0).toFixed(3));
+}
+
 class Navigator {
   // Private properties are used to avoid brand validations.
   #availableParallelism;
   #userAgent = `Node.js/${nodeVersion.slice(1, nodeVersion.indexOf('.'))}`;
+  #deviceMemory = getApproximatedDeviceMemory(getTotalMem());
 
   constructor() {
     if (arguments[0] === kInitialize) {
@@ -46,14 +70,23 @@ class Navigator {
   get userAgent() {
     return this.#userAgent;
   }
+
+  /**
+   * @returns {number}
+   */
+  get deviceMemory() {
+    return this.#deviceMemory;
+  }
 }
 
 ObjectDefineProperties(Navigator.prototype, {
   hardwareConcurrency: kEnumerableProperty,
   userAgent: kEnumerableProperty,
+  deviceMemory: kEnumerableProperty,
 });
 
 module.exports = {
+  getApproximatedDeviceMemory,
   navigator: new Navigator(kInitialize),
   Navigator,
 };

test/parallel/test-navigator.js

@@ -1,7 +1,12 @@
+// Flags: --expose-internals
+
 'use strict';
 
 require('../common');
 const assert = require('assert');
+const {
+  getApproximatedDeviceMemory,
+} = require('internal/navigator');
 
 const is = {
   number: (value, key) => {
@@ -10,6 +15,34 @@ const is = {
   },
 };
 
+assert.strictEqual(typeof navigator.deviceMemory, 'number');
+assert.ok(navigator.deviceMemory >= 0);
+
+assert.strictEqual(getApproximatedDeviceMemory(0), 0);
+assert.strictEqual(getApproximatedDeviceMemory(2 ** 27), 0.125);
+assert.strictEqual(getApproximatedDeviceMemory(2 ** 28), 0.25);
+assert.strictEqual(getApproximatedDeviceMemory(2 ** 29), 0.5);
+assert.strictEqual(getApproximatedDeviceMemory(2 ** 30), 1);
+assert.strictEqual(getApproximatedDeviceMemory(2 ** 31), 2);
+assert.strictEqual(getApproximatedDeviceMemory(2 ** 32), 4);
+assert.strictEqual(getApproximatedDeviceMemory(2 ** 33), 8);
+assert.strictEqual(getApproximatedDeviceMemory(2 ** 34), 16);
+assert.strictEqual(getApproximatedDeviceMemory(2 ** 35), 32);
+assert.strictEqual(getApproximatedDeviceMemory(2 ** 36), 64);
+assert.strictEqual(getApproximatedDeviceMemory(2 ** 37), 128);
+assert.strictEqual(getApproximatedDeviceMemory(2 ** 38), 256);
+assert.strictEqual(getApproximatedDeviceMemory(2 ** 39), 512);
+assert.strictEqual(getApproximatedDeviceMemory(2 ** 40), 1024);
+assert.strictEqual(getApproximatedDeviceMemory(2 ** 41), 2048);
+assert.strictEqual(getApproximatedDeviceMemory(2 ** 42), 4096);
+assert.strictEqual(getApproximatedDeviceMemory(2 ** 43), 8192);
+assert.strictEqual(getApproximatedDeviceMemory(2 ** 44), 16384);
+assert.strictEqual(getApproximatedDeviceMemory(2 ** 45), 32768);
+
+// https://github.com/w3c/device-memory#the-header
+assert.strictEqual(getApproximatedDeviceMemory(768 * 1024 * 1024), 0.5);
+assert.strictEqual(getApproximatedDeviceMemory(1793 * 1024 * 1024), 2);
+
 is.number(+navigator.hardwareConcurrency, 'hardwareConcurrency');
 is.number(navigator.hardwareConcurrency, 'hardwareConcurrency');
 assert.ok(navigator.hardwareConcurrency > 0);