Greeting by hour Euler Style math

const greet = who =>
    `Good ${(({ E:e,PI:π }) =>
        (T => [...T, ...T])(['night', 'evening', 'afternoon', 'morning'])
            .reduceRight(
                (Π, λ, i) => [
                    ...Π,
                    ...(`${[λ, ''][i >-~ π | '']}$`
                        .repeat(e +-~ i)
                        .match(/\w+/g) || [])
                ], [])
            .slice(+true))(Math)[
                        new Date().getHours()
                  ]} ${who||''}`
                  .trim();

// example
greet('you');