Abuse of reduce

fib.js

const range = (n) => [...Array(n).keys()];

const fib = (n) => range(n).slice(1).reduce( 
  (acc, value) => {
    console.log([acc[1], acc[0]+acc[1]]);
    return [acc[1], acc[0]+acc[1]]
  }
, [0, 1])[0]

fib(5)

mexicanWave.js

const mexicanWave = (str) => 
  str.split("").reduce( (acc, value, index, array) => {
    let wavedStr = array.concat();
    wavedStr[index] = wavedStr[index].toUpperCase()
    return acc.concat(wavedStr.join(""))
  }, [])


mexicanWave("lol")