JS Find Frequency of Letters in a Sentence

countLetterFreq.js

String.prototype.countLetterFreq = function () {
  return this
  .split('')
  .filter(l => l.trim())
  .map(l => l.toLowerCase())
  .reduce( (acc, cur) => { acc[cur] = (acc[cur] === undefined) ? 1 : acc[cur] + 1; return acc; }, {} ) 
}

// Usage: 
// > "This is a nice sentence".countLetterFreq()
// Returns:
// > { t: 2, h: 1, i: 3, s: 3, a: 1, n: 3, c: 2, e: 4 }