The maybe decoder

maybe.js

import { maybe, string, field } from 'jsonous';
        
const decoder = field('foo', maybe(string()));

decoder.decodeAny({ foo: 'bar' }) 
// --> Ok(Just('bar'))

decoder.decodeAny({ foo: null })  
// --> Ok(Nothing)

decoder.decodeAny({ foop: 'bar' })    
// --> Err('Expected to find an object with key 'foo'. Instead found {"foop":"bar"}')

const maybeDecoder = maybe(field('foo', string()));

maybeDecoder.decodeAny({ foop: 'bar' })    
// --> Ok(Nothing)