export function mapOutput<A, O, I, P>(
  codec: t.Type<A, O, I>,
  f: (p: O) => P,
  name: string = codec.name
): t.Type<A, P, I> { ... }

import * as t from 'io-ts'
import { mapOutput } from 'io-ts-types/lib/mapOutput'
import { optionFromNullable } from 'io-ts-types/lib/optionFromNullable'
import { none, some } from 'fp-ts/lib/Option'

// Input: t.Type<Option<number>, number | null, t.mixed>
const Input = optionFromNullable(t.number)

const toUndefined = <A>(x: A | null): A | undefined => (x === null ? undefined : x)

// Output: t.Type<Option<number>, number | undefined, t.mixed>
const Output = mapOutput(Input, toUndefined)

assert.strictEqual(Output.encode(none), undefined)
assert.strictEqual(Output.encode(some(1)), 1)