Node.js – Global Objects

These objects are global in nature and are available in all modules. There is no need to include these objects in our application, rather we can use them directly. These objects are modules, functions, strings, object etc.

Topics Covered :

• Console
• Process
• Scheduling Timers
  • setImmediate(callback[, …args])
  • setInterval(callback, delay[, …args])
  • setTimeout(callback, delay[, …args])
• Cancelling Timers
  • clearImmediate(immediateObject)
  • clearInterval(intervalObject)
  • clearTimeout(timeoutObject)
• Dirname : __dirname
• Filename : __filename
• Exports
• Module
• Require

Console

The console provides a simple debugging console. Used to print information on stdout and stderr. It uses built-in methods for printing informational, warning and error messages. It exports two specific components :

Console class : The class have methods like console.log(), console.error() and console.warn().
Global console instance: The instance is configured to write to process.stdout and process.stderr. The global console can be used without calling require(‘console’).

Example :

Create a file name text.js . Write the following code in text.js :

console.log('hello world');
// Prints hello world, to stdout 
console.log('hello %s', 'world'); 
// Prints hello world, to stdout 
console.error(new Error('Error')); 
// Prints Error: 'Error', to stderr 
const name = 'Global Objects'; 
console.warn(`Danger ${name}! Danger!`); 
// Prints: Danger Global Objects! Danger!, to stderr

Run Command :

C:\Users\Your Name>node text.js

Result :

hello world
hello world
Error: Error
Danger Global Objects! Danger!

There are two ways of using a console synchronous and asynchronous. When the destination is a file or a terminal synchronous console is used. When the destination is a pipe asynchronous console is used.

Inspector only methods

These methods does not display anything unless used in the inspector.

Process

• Used to get information over the current process.
• Provides multiple events related to process activities.

Process Events

It is an instance of EventEmitter. It emits the following events :

Example :

Create a file name text.js . Write the following code in text.js :

process.on('exit', function(code) {
 // The code below will never execute.
 setTimeout(function() {
    console.log("This will not run");
 }, 0);
 console.log('About to exit with code:', code);
});
console.log("Program Ended");

Run Command :

C:\Users\Your Name>node text.js

Result :

Program Ended
About to exit with code: 0

Process Properties

To better control system interactions, process properties are used :

Methods

Scheduling Timers

In Node.js a timer is a global function and an internal construct that calls a given function after a certain period of time. When a timer’s function is called it varies depending on which method was used to create the timer and what other work the Node.js event loop is doing.

setImmediate(callback[, …args])

callback     :     It is a callback function to call at the end of this turn of Event Loop

..args          :     Optional arguments to pass when the callback is called                                                                                                                 

• It sets the ‘immediate’ execution of the callback after I/O events callback returns an Immediate for clearImmediate().
• When there are multiple calls for setImmediate(), then the callback functions are queued for execution in the order in which they are created.
• For every event loop iteration, entire callback queue is processed.
• When an immediate timer is queued from inside an executing callback, then that timer will not be triggered until the next event loop iteration.

Example :

Create a file name text.js . Write the following code in text.js :

setImmediate(function () {
 console.log('immediate');
});

Run Command :

C:\Users\Your Name>node text.js

The output is printed after a little delay.

 The method has a custom variant for promises i.e. util.promisify() : 

Example :

Create a file name text.js . Write the following code in text.js :

const util = require('util');
const setImmediatePromise= util.promisify(setImmediate);

setImmediatePromise('aaa').then((value) => {
  // value === 'aaa' (passing values is optional)
  // This is executed after all I/O callbacks.
});

// or with async function
async function timerExample() {
  console.log('Before I/O callbacks');
  await setImmediatePromise();
  console.log('After I/O callbacks');
}
timerExample();

Run Command :

C:\Users\Your Name>node text.js

Result :

Before I/O callbacks
After I/O callbacks

Note : Callback has to be a function otherwise TypeError will be thrown.

setInterval(callback, delay[, …args])

callback     :     It is a callback function to call when the timer pass.

delay          :     It is of type number. The number of milliseconds to wait before calling the callback.

..args          :     Optional arguments to pass when the callback is called.   

Run callback repeatedly after at least delay of milliseconds. OS timer granularity and system load may affect actual delay. Returns a Timeout for use with clearInterval(). A timer cannot span larger than 2147483647 or less than 1, otherwise, the delay will be set to 1.

Example :

Create a file name text.js . Write the following code in text.js :

function printHello(){
   console.log( "Hello, World!");
}
// Call the above function after 2 seconds
setInterval(printHello, 2000);

Run Command :

C:\Users\Your Name>node text.js

The output is printed after delay.

setTimeout(callback, delay[, …args])

callback     :     It is a callback function to call when the timer pass.

delay          :     It is of type number. The number of milliseconds to wait before calling the callback.

…args         :     Optional arguments to pass when the callback is called.

Executes a one-time callback after the delay of milliseconds. Returns a Timeout for clearTimeout(). A timer cannot span larger than 2147483647 or less than 1, otherwise, the delay will be set to 1.

Example :

Create a file name text.js . Write the following code in text.js :

function printHello(){
   console.log( "Hello, World!");
}
//Call above function after 2 seconds
setTimeout(printHello, 2000);

Run Command :

C:\Users\Your Name>node text.js

The output is printed after a little delay.

 The method has a custom variant for promises i.e. util.promisify() : 

Example :

Create a file name text.js . Write the following code in text.js :

const util = require('util');
const setTimeoutPromise = util.promisify(setTimeout);

setTimeoutPromise(40, 'foobar').then((value) => {
  // value === 'foobar' (passing values is optional)
  // This is executed after about 40 milliseconds.
console.log(value);
});

Run Command :

C:\Users\Your Name>node text.js

The output is printed after delay.

Note: Callback has to be a function otherwise TypeError will be thrown.

Cancelling Timers

The setImmediate(), setInterval() and setTimeout() methods return objects that represent the scheduled timers. Cancelling timers can be used to cancel the timer and stop triggering.

clearImmediate(immediateObject)

The clearTimeout(t) global function is used to stop a timer returned by the setImmediate() function.

Example :

Create a file name text.js . Write the following code in text.js :

function printHello(){
   console.log( "Hello, World!");
}

// Call above function after 2 seconds
var t = setImmediate(printHello, 2000);

// Clear the timer
clearTimeout(t);

Run Command :

C:\Users\Your Name>node text.js

You will not find anything printed in the output.

clearInterval(intervalObject)

The clearTimeout(t) global function is used to stop a timer returned by the setInterval() function.

Example :

Create a file name text.js . Write the following code in text.js :

function printHello(){
   console.log( "Hello, World!");
}

// Call above function after 2 seconds
var t = setInterval(printHello, 2000);

// Clear the timer
clearTimeout(t);

Run Command :

C:\Users\Your Name>node text.js

You will not find anything printed in the output.

clearTimeout(timeoutObject)

The clearTimeout(t) global function is used to stop a timer returned by the setTimeout() function.

Example :

Create a file name text.js . Write the following code in text.js :

function printHello(){
   console.log( "Hello, World!");
}

// Call above function after 2 seconds
var t = setTimeout(printHello, 2000);

// Clear the timer
clearTimeout(t);

Run Command :

C:\Users\Your Name>node text.js

You will not find anything printed in the output.

The following variables exist only in the scope of modules, they may appear to be global but are not :

• __dirname
• __filename
• exports
• module
• require()

__dirname

The __dirname is same as the path.dirname() of the __filename. It is of type string. It specifies the directory name of the current module.

Example :

Create a file name test.js . Write the following code in test.js :

// Prints the value of __dirname
console.log( __dirname );

Run Command :

C:\Users\Your Name>node dirname.js

__filename

The __filename is the resolved absolute path of the current module. The value within a module is the path to that module file. It is of type string. It specifies the filename of the code being executed.

Example :

Create a file name test.js . Write the following code in test.js :

//Print the value of __filename
console.log( __filename );

Run Command :

C:\Users\Your Name>test.js

 Exports

exports is a shortcut reference to the module.exports . It’s a variable available within the file-level scope of the module and assigned the value of module.exports before the module are appraised.

It allows a shortcut to the module.exports, so that module.exports.f = … can be written as exports.f = … . However, if a new value is assigned to exports it is no longer bound to the module.exports :

// Exported from require of module
module.exports.hello = true; 
// Not exported, only available in the module
exports = { hello: false };

Module

The fundamental node building block is called a module which maps directly to a file. A module is of type object. The module free variable is a reference to the object representing the current module. a module is not actually a global but rather local to each module. The module Object free variable is a reference to the object representing the current module.

Built-in modules which you can use without any further installation :

Require()

Require is used to load a module and its return value is typically assigned to a variable. It reads a javascript file, executes the file and then proceeds to return the exports object.

Syntax :

require(‘module_name’)

require.cache

Modules are cached in this object when they are required.

require.resolve(request[, options])

request: It is of type string. Contains the path of the module to resolve.

options: It is of type object. Contains path to resolve module location from.

require.resolve.paths(request)

request: It is of type string. Contains the module path whose lookup paths are being retrieved.

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