The global object in Node.js

Overview of Node.js Global Objects

In the Node.js runtime environment, there are some global objects that can be used directly without requiring them. These objects are shared across different modules and provide basic API functionality. Similar to the window object in browsers, Node.js's global objects are specifically designed for server-side environments.

The global Object

global is the top-level global object in Node.js. All global variables (except module itself) are properties of the global object. In the REPL environment, you can view all properties by directly entering global:

console.log(global.process === process);  // true
console.log(global.setTimeout === setTimeout);  // true

Key characteristics to note:

1. Variables declared with var are not added to the global object.
2. Variables declared with let or const do not appear on the global object.
3. Assigning a value to an undeclared variable makes it a property of global.

var foo = 'var';
let bar = 'let';
baz = 'global'; 

console.log(global.foo); // undefined
console.log(global.bar); // undefined  
console.log(global.baz); // 'global'

The process Object

The process object provides an interface for interacting with the current Node.js process. Common functionalities include:

Environment Variable Operations

// Get environment variables
console.log(process.env.PATH);

// Set environment variables
process.env.NODE_ENV = 'production';

Process Control

// Exit the process
process.exit(1);

// Listen for exit events
process.on('exit', (code) => {
  console.log(`Exit code: ${code}`);
});

Standard Input/Output

process.stdin.on('data', (data) => {
  process.stdout.write(`Received: ${data}`);
});

Performance Analysis

// Memory usage
console.log(process.memoryUsage());

// CPU usage
console.log(process.cpuUsage());

The console Object

Node.js's console is similar to the browser's but has some extensions:

Formatted Output

console.log('Count: %d', 10);
console.log('Object: %j', {key: 'value'});

Timing Functions

console.time('loop');
for(let i=0; i<1000000; i++){}
console.timeEnd('loop');  // loop: 2.123ms

Stack Tracing

console.trace('Current location');

Timer-Related Functions

Node.js provides four types of timer functions:

// One-time timer
const timeout = setTimeout(() => {
  console.log('Executes after 1 second');
}, 1000);

// Periodic timer  
const interval = setInterval(() => {
  console.log('Executes every second');
}, 1000);

// Immediate execution
const immediate = setImmediate(() => {
  console.log('Executes in the next event loop');
});

// Clean up timers
clearTimeout(timeout);
clearInterval(interval); 
clearImmediate(immediate);

The Buffer Class

Buffer is a global class for handling binary data:

// Create a Buffer
const buf1 = Buffer.alloc(10); // Initialize 10 bytes
const buf2 = Buffer.from('hello');

// Write data
buf1.write('Node.js');

// Read data
console.log(buf2.toString('utf8', 0, 2)); // 'he'

// Convert formats
const buf3 = Buffer.from('74657374', 'hex');
console.log(buf3.toString()); // 'test'

__filename and __dirname

These variables provide information about the current module's file path:

console.log(__filename); // Absolute path of the current file
console.log(__dirname);  // Absolute path of the current directory

// Example of path joining
const path = require('path');
const fullPath = path.join(__dirname, 'config.json');

module, exports, and require

Although they appear to be global variables, they are actually local to each module:

// Two ways to use module.exports
module.exports = { key: 'value' };
exports.key = 'value'; 

// Using require
const fs = require('fs');
const myModule = require('./my-module');

Other Important Global Objects

The URL Class

const myURL = new URL('https://example.com/path?query=123');
console.log(myURL.searchParams.get('query')); // '123'

TextEncoder/TextDecoder

const encoder = new TextEncoder();
const decoder = new TextDecoder();

const uint8Array = encoder.encode('Hello');
console.log(decoder.decode(uint8Array)); // 'Hello'

performance

const { performance } = require('perf_hooks');

performance.mark('start');
// Perform some operations
performance.mark('end');

performance.measure('Duration', 'start', 'end');
console.log(performance.getEntriesByName('Duration'));

Global Objects and the Module System

Node.js's module system affects the visibility of global variables:

1. Each module has its own scope; variables declared with var do not pollute global.
2. Modules loaded via require are cached in the require.cache object.
3. The true global object can be accessed via globalThis (ES2020 standard).

// Share data across modules
global.sharedData = { count: 0 };

// Check module cache
console.log(require.cache);