transformCollection.js

const UNGROUPED_KEY = 'ungrouped';

const transformCollection = (collection, keyField) => collection.reduce((acc, curValue) => {
	const DESIRED_KEY = curValue[keyField];
  if (acc[`${DESIRED_KEY}`.toLowerCase()]) {
    acc[`${DESIRED_KEY}`.toLowerCase()] = [...acc[`${DESIRED_KEY}`.toLowerCase()], curValue];
  } else {
    if (DESIRED_KEY) {
      acc[`${DESIRED_KEY}`.toLowerCase()] = [curValue];
    } else {
    	acc[UNGROUPED_KEY] = acc[UNGROUPED_KEY] ? [...acc[UNGROUPED_KEY], curValue] : [curValue];
    }
  }

  return acc;
}, {});

console.log(transformCollection(data, 'name'));
console.log(transformCollection(data, 'unit'));
console.log(transformCollection(data, 'value'));
console.log(transformCollection(data, 'threshold'));