Test Driven Development (TDD)

Write a simple test

Let's suppose we want to add a SmartThermostat class. We begin by writing one simple test.

import SmartThermostat from '../models/SmartThermostat.js'
import SmartDevice from '../models/SmartDevice.js'
import { describe, it } from 'mocha'
import assert from 'assert'

describe('The SmartThermostat class', function () {
  it('should be a SmartDevice', function () {
    const thermostat = new SmartThermostat()
    assert.strictEqual(thermostat instanceof SmartDevice, true)
  })
})

Make it pass

We run our tests and observe they are failing. Then, write just enough code to make the test pass:

import SmartDevice from './SmartDevice.js'

class SmartThermostat extends SmartDevice {}

export default SmartThermostat

We have satisfied the test, and we can move on.

Add another test

We add a new test to our test suite, representing the functionality we want to work on.

import SmartThermostat from '../models/SmartThermostat.js'
import SmartDevice from '../models/SmartDevice.js'
import { describe, it } from 'mocha'
import assert from 'assert'

describe('The SmartThermostat class', function () {
  it('should be a SmartDevice', function () {
    const thermostat = new SmartThermostat()
    assert.strictEqual(thermostat instanceof SmartDevice, true)
  })

  describe('the temperature', () => {
    const thermostat = new SmartThermostat()

    it('should be 18 by default', () => {
      assert.strictEqual(thermostat.temperature, 18)
    })
  })
})

Make it pass

import SmartDevice from './SmartDevice.js'

class SmartThermostat extends SmartDevice {
  constructor() {
    super()
    this.temperature = 18
  }
}

export default SmartThermostat

Keep going

TDD is iterative. It encourages developers to think carefully about what the code should do before writing it. The goal is to gradually produce working code along with a suite of tests which verify its correctness. If anybody else tries to change the code, they can run the tests to ensure that their changes haven't broken anything.