https://leetcode.com/problems/find-the-difference/

findTheDifference.js

const findTheDifference =
    ( s , t ) =>
        String.fromCharCode(
            foldMap( Sum , toCharCode , t )
            - foldMap( Sum , toCharCode , s ) )

const toCharCode = char => char.charCodeAt()

const foldMap =
    ( M , f , xs ) => {
        let y = M.empty
        for ( const x of xs )
            y = M.concat( y , f( x ) )
        return y
    }

const Sum =
    { concat : ( x , y ) => x + y
    , empty : 0
    }