GroupBy Function

Data Manipulation2 functions

Utility Overview

Details

Groups array items by a key selector function or property name

Category:Data Manipulation

Functions:groupBy, groupByProperty

Performance:

- Time: O(n) single pass through array - Space: O(n) for grouped result structure

Concepts:

array manipulationdata groupingfunctional programmingdata transformation

Implementation

groupBy

Time: O(n) | Space: O(n)

export function groupBy<T, K extends string | number | symbol>(
  array: T[],
  keySelector: (item: T) => K
): Record<K, T[]> {
  if (!Array.isArray(array)) {
    throw new Error('First argument must be an array');
  }
  
  if (typeof keySelector !== 'function') {
    throw new Error('Second argument must be a function');
  }
  
  const result = {} as Record<K, T[]>;
  
  for (const item of array) {
    const key = keySelector(item);
    if (!result[key]) {
      result[key] = [];
    }
    result[key].push(item);
  }
  
  return result;
}

Usage Examples

Group objects by property

const users = [
  { name: 'John', role: 'admin', age: 30 },
  { name: 'Jane', role: 'user', age: 25 },
  { name: 'Bob', role: 'admin', age: 35 }
];

const byRole = groupByProperty(users, 'role');
// { admin: [John, Bob], user: [Jane] }