group-by.js

const data = [
  { key: 'a', val: 'foo' },
  { key: 'b', val: 'bar' },
  { key: 'c', val: 'baz' },
  { key: 'a', val: 'qux' },
  { key: 'b', val: 'quxx' }
];

function groupBy(key, arr) {
  const result = {};
  for (const el of arr) {
    const group = result[el[key]] ??= [];
    group.push(el);
  }
  return result;
}

console.log(groupBy('key', data));