Group array of strings by first letter.js

// Write a function that takes an array of strings as argument
// Group those strings by their first letter
// Return an object that contains properties with keys representing first letters
// The values should be arrays of strings containing only the corresponding strings
// For example, the array ['Alf', 'Alice', 'Ben'] should be transformed to
// { a: ['Alf', 'Alice'], b: ['Ben']}

function myFunction(arr) {
  return arr.reduce((pv, cv) => {
    const c = cv[0].toLowerCase();
    return {...pv, [c]: [...(pv[c] || []), cv]}
  }, {})
}