JavaScript - Ayuda con un código

let frase = "Este es un lindo ejercicio";

let upper = frase.toUpperCase();

let firstWord = upper.at(0)

let arr = upper.split(" ");

var newArray = [];

arr.map(word => {

  if (word.startsWith(firstWord)) {

    newArray.push(word.split("").reverse().join(""))

  } else { newArray.push(word) }

});

let finalFrase = newArray.join(" ")

console.log(finalFrase.toLowerCase())