partition.js

function partition(inputArray, callback) {
  const result = {};
  for (const [indexOfValue, value] of inputArray.entries()) {
    const propertyKey = callback(value, indexOfValue);
    if (propertyKey === null || propertyKey === '') {
      continue;
    }
    if (!{}.hasOwnProperty.call(result, propertyKey)) {
      result[propertyKey] = [];
    }
    result[propertyKey].push(value);
  }
  return result;
}


const arr = [1, -5, 4, 5, -2]
const {negative, nonNegative} = partition(arr,
  elem => elem < 0 ? 'negative' : 'nonNegative');
console.log(negative);
  // [ -5, -2 ]
console.log(nonNegative);
  // [ 1, 4, 5 ]


const input = [
  {name: 'John', isLocal: true},
  {name: 'Jane', isLocal: false},
  {name: 'Charlie', isLocal: true}
];
const {local, global} = partition(input,
  (elem) => elem.isLocal ? 'local' : 'global');
console.log(local);
  // [ { name: 'John', isLocal: true }, { name: 'Charlie', isLocal: true } ]
console.log(global);
  // [ { name: 'Jane', isLocal: false } ]

Nice gist! Any reason not to include undefined as a "negative" result on line 5?

I don't think there is a specific reason for this. You could include it as well. At the same time you could also exclude null or '' from the check, resulting in an object that could have "null" or "''" as a key. It's a matter of taste I guess. Or maybe there is another reason that I cannot foresee at the moment.