micro-test-engine.js

const assert = f => ([a, b]) =>
  (x => x === b
    ? { passed: true }
    : { 
      failed: true, 
      message: `- expected f(${a}) to be ${b}, but was ${x}` 
    }
  )(f(a))

const runTests = (cases, f) => {
  const results = cases.map(assert(f))
  const [passed, failed] = ['passed', 'failed'].map(
    p => results.filter(x => x[p])
  )
  
  return { passed, failed }
}

const failureSummary = failed => [
  `${failed.length} test${failed.length > 1 ? 's' : ''} have failed:`, 
  failed.map(x => x.message).join('\n\t')
].join('\n\t')

const report = ({ passed, failed }) => {
  console.log(
    `${passed.length} tests passed`
  )
  
  if (failed.length) {
    const summary = failureSummary(failed)
    console.error(summary)
  }
}