Cucumber is a tool that supports Behaviour-Driven Development(BDD). If you’re new to Behaviour-Driven Development read our BDD introduction first.

What is Cucumber?

Ok, now that you know that BDD is about discovery, collaboration and examples (and not testing), let’s take a look at Cucumber.

Cucumber reads executable specifications written in plain text and validates that the software does what those specifications say. The specifications consists of multiple examples, or scenarios. For example:

Scenario: Breaker guesses a word
  Given the Maker has chosen a word
  When the Breaker makes a guess
  Then the Maker is asked to score

Each scenario is a list of steps for Cucumber to work through. Cucumber verifies that the software conforms with the specification and generates a report indicating ✅ success or ❌ failure for each scenario.

In order for Cucumber to understand the scenarios, they must follow some basic syntax rules, called Gherkin.

What is Gherkin?

Gherkin is a set of grammar rules that makes plain text structured enough for Cucumber to understand. The scenario above is written in Gherkin.

Gherkin serves multiple purposes:

• Unambiguous executable specification
• Automated testing using Cucumber
• Document how the system actually behaves

The Cucumber grammar exists in different flavours for many spoken languages so that your team can use the keywords in your own language.

Gherkin documents are stored in .feature text files and are typically versioned in source control alongside the software.

See the Gherkin reference for more details.

What are Step Definitions?

Step definitions connect Gherkin steps to programming code. A step definition carries out the action that should be performed by the step. So step definitions hard-wire the specification to the implementation.

┌────────────┐                 ┌──────────────┐                 ┌───────────┐
│   Steps    │                 │     Step     │                 │           │
│ in Gherkin ├──matched with──>│ Definitions  ├───manipulates──>│  System   │
│            │                 │              │                 │           │
└────────────┘                 └──────────────┘                 └───────────┘

Step definitions can be written in many programming languages. Here is an example using JavaScript:

When("{maker} starts a game", function(maker) {
  maker.startGameWithWord({ word: "whale" })
})