Introduction

Grouping an array of objects means building buckets keyed by one property or derived value. In JavaScript, the efficient baseline is a single pass through the array, usually with reduce, a for...of loop, or a Map when the grouping key should not be forced into plain object-property semantics.

Single-Pass Grouping With reduce

A common and efficient pattern is to walk the array once and append each item into the right bucket.

1const employees = [
2  { name: 'Alice', department: 'Engineering' },
3  { name: 'Bob', department: 'HR' },
4  { name: 'Charlie', department: 'Engineering' },
5  { name: 'David', department: 'Marketing' }
6]
7
8const grouped = employees.reduce((acc, employee) => {
9  const key = employee.department
10  if (!acc[key]) {
11    acc[key] = []
12  }
13  acc[key].push(employee)
14  return acc
15}, {})
16
17console.log(grouped)

This is O(n) time because every item is processed once, and the bucket lookup is constant time in ordinary cases.

for...of Is Often Just as Good

If performance and readability matter equally, a normal loop is perfectly respectable.

1const grouped = {}
2for (const employee of employees) {
3  const key = employee.department
4  if (!grouped[key]) {
5    grouped[key] = []
6  }
7  grouped[key].push(employee)
8}

Some teams prefer this style because it is more direct and easier to debug step by step. The algorithmic cost is the same as the reduce version.

When Map Is Better Than Plain Objects

If grouping keys are not naturally strings, or if you want to avoid prototype-related edge cases, use Map.

1const grouped = new Map()
2
3for (const employee of employees) {
4  const key = employee.department
5  if (!grouped.has(key)) {
6    grouped.set(key, [])
7  }
8  grouped.get(key).push(employee)
9}
10
11console.log(grouped)

Map is often the better abstraction when keys may be numbers, objects, or values that should not be coerced to object property names.

Efficiency Is Not Only About Syntax

The “most efficient” method is not really about whether you used reduce or a loop. It is about avoiding extra passes, repeated filtering, or nested scans.

This is inefficient because it rescans the array repeatedly:

const departments = [...new Set(employees.map(e => e.department))]
const grouped = departments.map(dep => employees.filter(e => e.department === dep))

That pattern is much more expensive on large arrays because each group requires another walk through the full collection.

Reusable Grouping Helper

If you group data often, wrap the one-pass approach in a helper.

1function groupBy(array, keySelector) {
2  const result = new Map()
3  for (const item of array) {
4    const key = keySelector(item)
5    if (!result.has(key)) {
6      result.set(key, [])
7    }
8    result.get(key).push(item)
9  }
10  return result
11}
12
13const byDepartment = groupBy(employees, e => e.department)
14console.log(byDepartment.get('Engineering'))

This keeps the algorithm efficient while making the call sites cleaner.

Common Pitfalls

• Re-grouping with repeated filter calls instead of doing one pass through the array.
• Treating reduce as automatically faster than a simple loop when the real performance difference is usually negligible.
• Using plain objects when a Map would represent the grouping keys more accurately.
• Forgetting that object keys are strings, which can surprise you with non-string grouping values.
• Optimizing syntax choice before confirming that grouping logic is actually a performance bottleneck.

Summary

• Efficient grouping means building buckets in one pass over the data.
• 'reduce, for...of, and Map-based approaches can all do that in O(n) time.'
• The biggest inefficiency is repeated rescanning, not the choice between loop and reduce.
• Use Map when keys should not be forced into plain object properties.
• Pick the one-pass style that your team can read and maintain easily.