JSFiddle - React, Tailwind, and code Playground

JavaScript

const values = [
  ["A", 1],
  ["B", 2],
  ["A", 3],
  ["A", 4],
  ["B", 5],
  ["C", 6]
]
var grouped = {};

for(let elem of values){
    if(grouped.hasOwnProperty(elem[0])){
        grouped[elem[0]].push(elem); 
    } else {
        grouped[elem[0]] = [elem];
    }
}

console.log(grouped);