util.inherits() Method in Node.js

Introduction

The util.inherits() method in Node.js is part of the Utility Module and is used to establish classical inheritance between constructor functions. It links the prototype of one constructor function to another, allowing objects created from the child constructor to inherit methods from the parent.

Syntax

util.inherits(childConstructor, parentConstructor)

This function connects the prototype chain of childConstructor to parentConstructor, allowing inheritance of prototype methods.

Parameters

• childConstructor: The constructor function that will inherit from the parent.
• parentConstructor: The constructor function whose prototype methods will be inherited.

Important Note

util.inherits() only links prototype methods, not properties defined inside the parent constructor using this. For full inheritance of properties and methods, consider using ES6 classes.

Example

const util = require('util');

// Parent constructor
function Person() {
  this.type = 'Human';
}

Person.prototype.sayHello = function() {
  console.log('Hello from Person');
};

// Child constructor
function Student() {
  this.level = 'Graduate';
}

// Establish inheritance
util.inherits(Student, Person);

// Test
const stu = new Student();
stu.sayHello(); // Inherited from Person
console.log(stu.level);
  

Output

Hello from Person
Graduate
  

Use Cases

• Useful for creating inheritance structures in older Node.js codebases.
• Helps share common behaviors across constructor-based objects.
• Good when not using ES6 class syntax but still need inheritance.

Conclusion

The util.inherits() method offers a way to implement prototype-based inheritance in Node.js using traditional constructor functions. While modern JavaScript uses class and extends, this method remains useful in legacy code and simpler prototypes.