vue-cli/packages/@vue/cli-service/__tests__/generator.spec.js

const generateWithPlugin = require('@vue/cli-test-utils/generateWithPlugin')

test('node sass (legacy)', async () => {
  const { pkg, files } = await generateWithPlugin([
    {
      id: '@vue/cli-service',
      apply: require('../generator'),
      options: {
        cssPreprocessor: 'sass'
      }
    }
  ])

  expect(files['src/App.vue']).toMatch('<style lang="scss">')
  expect(pkg).toHaveProperty(['devDependencies', 'node-sass'])
})

test('node sass', async () => {
  const { pkg, files } = await generateWithPlugin([
    {
      id: '@vue/cli-service',
      apply: require('../generator'),
      options: {
        cssPreprocessor: 'node-sass'
      }
    }
  ])

  expect(files['src/App.vue']).toMatch('<style lang="scss">')
  expect(pkg).toHaveProperty(['devDependencies', 'node-sass'])
})

test('dart sass', async () => {
  const { pkg, files } = await generateWithPlugin([
    {
      id: '@vue/cli-service',
      apply: require('../generator'),
      options: {
        cssPreprocessor: 'dart-sass'
      }
    }
  ])

  expect(files['src/App.vue']).toMatch('<style lang="scss">')
  expect(pkg).toHaveProperty(['devDependencies', 'sass'])
})