Array to grouped

JavaScript

const values = [
  ["A", 1],
  ["B", 2],
  ["A", 3],
  ["A", 4],
  ["B", 5],
  ["C", 6]
]

const grouped = {}

for (let i = 0; i < values.length; i++) {
  if (grouped[values[i][0]]){
      grouped[values[i][0]].push(values[i]);
    } else {
      grouped[values[i][0]] = values[i];
    }
}

console.log(grouped);