Group strings starting with similar number in JavaScript

Suppose, we have an array of Numbers represented by strings like this −

const arr = ["1.1","1.2","1.3","2.1","2.2","3.1","3.2","3.3","4.1","4.2"];

We are required to write a JavaScript function that takes in one such array and groups all the strings starting with same number in a common subarray.

Therefore, the output of our function should look like −

const output = [["1.1","1.2","1.3"],["2.1","2.2"],["3.1","3.2","3.3"],["4.1","4.2"]];

Example

The code for this will be −

const arr = ["1.1","1.2","1.3","2.1","2.2","3.1","3.2","3.3","4.1","4.2"];
const groupSimilarStarters = arr => {
   let res = [];
   res = arr.reduce((acc, val, ind) => {
      const firstChar = el => {
         return (el || '').split('.')[0];
      }
      if(firstChar(val) === firstChar(arr[ind - 1])){
         acc[acc.length - 1].push(val);
      }else{
         acc.push([val]);
      };
      return acc;
   }, []);
   return res;
}
console.log(groupSimilarStarters(arr));

Output

The output in the console −

[
   [ '1.1', '1.2', '1.3' ],
   [ '2.1', '2.2' ],
   [ '3.1', '3.2', '3.3' ],
   [ '4.1', '4.2' ]
]