Transform a list of items into a smaller one (containing X elements). Keep the X-1 first items and concatenate the remaining into one single item.

summarize-list.ts

/**
 * Keep the @param nb first values of the list @param values.
 * Apply the @param reduceFn function to "concatenate" the remaining values.
 * @param values List of values/objects.
 * @param nb Final number of values in the list.
 * @param orderingFn Function to use to order the list.
 * @param reduceFn Reduction function to group exceeding items.
 */
export function summarizeList<T>(values: T[], nb: number = 10, orderingFn: (a: T, b: T) => number, reduceFn: (list: T[]) => T) {
  // From nb+ values, we trigger the grouping
  if (values.length > nb) {
    // Sort the list.
    const orderedValues = values.sort(orderingFn);

    // nb-1 => the last item is the grouping item
    const smallValues = orderedValues.slice(nb - 1);

    // Build grouping item.
    const grouped = reduceFn(smallValues);

    // Remove the small values from the initial list.
    smallValues.forEach(x => values.splice(values.indexOf(x), 1));

    // Add the grouping item to the list.
    values.push(grouped);
  }

  return values;
}

// Usage: Get 3 highest values and group the remaining (useful for pie charts to group small values).
const arr = [
  { value: 1 },
  { value: 2 },
  { value: 3 },
  { value: 4 },
  { value: 5 },
  { value: 6 },
];

const summarizedList = summarizeList(
  arr,
  4,
  (a, b) => Math.abs(b.value) - Math.abs(a.value),
  (remaining) => ({ value: remaining.reduce((s, i) => s+i.value, 0) })
);