Mocking with generators in JavaScript

gistfile1.txt

function delay(fn) {
  return function(...args) {
    return [fn, args]
  }
}

function a(arg) {
  // performs some IO operation that have to be mocked for testing
  return arg + 10
}

function b(arg) {
  // performs some IO operation that have to be mocked for testing
  return arg*3
}

function* calculate(arg) {
  const result_from_a = yield delay(a)(arg)
  const result_from_b = yield delay(b)(result_from_a * 2)
  return result_from_b
}

function run(generator) {
  let result
  while(true) {
    let {value, done} = generator.next(result)
    if(done) {
      return value
    } 
    const [fn, args] = value
    // TODO try, use generator.throw in catch block
    result = fn(...args)
  }
}

function run_expecting_effects(generator, expected_effects) {
  let effect_index = 0
  let result
  while(true) {
    let {value, done} = generator.next(result)
    if(done) {
      // TODO check that no more expects left
      return value
    } 
    const [fn, args] = value
    const effect = expected_effects[effect_index++]
    if(
      fn.name != effect.name
      ||
      JSON.stringify(effect.args) != JSON.stringify(args)
    ) {
      console.error({fn, args, effect})
      throw new Error('unexpected effect')
    }
    result = effect.result
  }
}

console.log('run', run(calculate(3))) // returns 78

console.log(
  'run_expecting_effects',
  run_expecting_effects(
    calculate(3),
    [
      {name: 'a', args: [3], result: 13},
      {name: 'b', args: [26], result: 78},
    ]
  )
) // returns 78