ninjajs-ua-rev.js

// Reverse only the letters of words in a sentence

// A helper function to reverse words
const reverseWord = str => str.split('').reverse().join('')

// Actual function
const reverseLettersInSentence = sentence => sentence
  .split(' ')
  .map(reverseWord)
  .join(' ')

// Execution
reverseLettersInSentence('Hello World')
// Output
"olleH dlroW"