Cucumber.js is a popular JavaScript library that allows developers to write executable specifications in a human-readable format. These specifications are called feature files, and they describe the behavior of a software application in a way that is easy to understand for both technical and non-technical stakeholders.

Cucumber.js is an implementation of the Cucumber framework, which was originally developed in the Ruby programming language. However, Cucumber.js has been adapted to work with JavaScript and Node.js, making it a popular choice for web developers who use JavaScript as their primary programming language.

One of the key benefits of using Cucumber.js is that it helps bridge the gap between business stakeholders and developers. Business stakeholders, such as product owners and project managers, can use feature files to communicate their requirements in a way that is easy to understand. Developers, on the other hand, can use these feature files to write automated tests that ensure the software meets these requirements.

To get started with Cucumber.js, developers need to install the library using npm, the package manager for Node.js. Once installed, they can create feature files using the Gherkin syntax, which is a plain-text language that describes the behavior of a software application.
Here is an example of a simple feature file written in Gherkin:

Feature: Login
As a user
I want to be able to login
So that I can access my account
Scenario: Successful login
  Given I am on the login page
  When I enter my username and password
  And I click the login button
  Then I should see the dashboard

This feature file describes a scenario where a user wants to login to their account. The scenario has several steps, each of which is defined using a Gherkin keyword (Given, When, Then, etc.) and a description of the action that should be taken.

To automate this scenario using Cucumber.js, developers would create a step definition file that defines the behavior of each step. Here is an example of a step definition file for the Login feature:

const { Given, When, Then } = require('cucumber');
Given('I am on the login page', function () {
// Navigate to the login page
});
When('I enter my username and password', function () {
// Enter the username and password
});
When('I click the login button', function () {
// Click the login button
});
Then('I should see the dashboard', function () {
// Verify that the dashboard is displayed
});

This step definition file uses the Cucumber.js API to define the behavior of each step. The code inside each step definition function is executed when the corresponding step is encountered during the execution of the automated test.

By using Cucumber.js, developers can write automated tests that are easy to read and understand, even for non-technical stakeholders. This helps ensure that the software meets the requirements of the business, and can help reduce the number of bugs and issues that arise during development.