How to use the rambda.reduce function in rambda

it('fallback with index', () => {
    const result = reduce((acc, val, i) => {
      acc // $ExpectType number
      i // $ExpectType number
      return acc + val
    }, 1,[ 1, 2, 3 ])

    result // $ExpectType number
  });

it('with two types', () => {
    const result = reduce(
      (acc, elem) => {
        acc // $ExpectType string
        elem // $ExpectType number

        return `${acc}${elem}`
      },
      'foo',
      [ 1, 2, 3 ]
    )

    result // $ExpectType string
  });

export default function getAccount(isDemo) {
  const prefix = isDemo ? 'DEMO' : 'LIVE'
  const keys = [ 'apiKey', 'username', 'password' ]
  return reduce((result, key) => {
    result[key] = path(`env.${prefix}_${constantCase(key)}`, process)
    return result
  }, { isDemo }, keys)
}

add('rambda', () => {
    RB.reduce((a, b) => a + b, 0)(input)
  }),

function covertToArray(registry) {
  return reduce(
    (accum, key) => {
      accum.push(registry[key])
      return accum
    },
    [],
    keys(registry)
  )
}

function injectSubtree(registry, selector) {
  return compose(
    partial(appendToDOM, [selector]),
    reduce(
      (arrayOfElements, item) => (
        arrayOfElements.push(item.element), arrayOfElements
      ),
      []
    ),
    filter(propEq("selector", selector))
  )(covertToArray(registry))
}