Group array of strings by first letter

Write a function that takes an array of strings as an argument and group those strings by their first letter. It should return an object that contains properties with keys representing the first letter. 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’]}

const getNewObj = (arr) => {
    const result = {};
   for(let i = 0; i < arr.length ; i++) {
       const item = arr[i];
       const firstLetter = item[0]
       if(!result[firstLetter]) {
           result[firstLetter] = [];
       }

       if(result[firstLetter].indexOf(item) < 0) {
           result[firstLetter].push(item);
       }
   }
   return result;
}

const inputArr = ['Berlin', 'Paris', 'Prague'];
console.log(getNewObj(inputArr));

// Output: 
{
    "B": [
        "Berlin"
    ],
    "P": [
        "Paris",
        "Prague"
    ]
}