Remove Duplicates from Array

Removing duplicates from arrays is a common task in JavaScript. Here are several effective methods to accomplish this.

Method 1: Using Set (Recommended)

The simplest and most performant way to remove duplicates is using a Set:

const array = [1, 2, 2, 3, 4, 4, 5]
const uniqueArray = [...new Set(array)]
console.log(uniqueArray) // [1, 2, 3, 4, 5]

Pros:

• Clean and concise
• Good performance O(n)
• Works with primitive values

Cons:

• Only works with primitive values (numbers, strings, booleans)
• Objects are compared by reference

Method 2: Using Filter

For more complex scenarios or when you need to keep the first occurrence:

const array = [1, 2, 2, 3, 4, 4, 5]
const uniqueArray = array.filter((value, index, self) => {
  return self.indexOf(value) === index
})
console.log(uniqueArray) // [1, 2, 3, 4, 5]

Pros:

• More flexible for custom logic
• Easy to understand

Cons:

• Less performant O(n²) due to indexOf

Method 3: Using Reduce

A functional approach using reduce:

const array = [1, 2, 2, 3, 4, 4, 5]
const uniqueArray = array.reduce((acc, current) => {
  if (!acc.includes(current)) {
    acc.push(current)
  }
  return acc
}, [])
console.log(uniqueArray) // [1, 2, 3, 4, 5]

Pros:

• Functional programming style
• Can be combined with other transformations

Cons:

• Less performant due to includes
• More verbose

Method 4: For Objects (Using Map)

When dealing with objects, you need to compare by a specific property:

const users = [
  { id: 1, name: 'Alice' },
  { id: 2, name: 'Bob' },
  { id: 1, name: 'Alice' },
  { id: 3, name: 'Charlie' }
]

const uniqueUsers = Array.from(new Map(users.map((user) => [user.id, user])).values())

console.log(uniqueUsers)
// [
//   { id: 1, name: 'Alice' },
//   { id: 2, name: 'Bob' },
//   { id: 3, name: 'Charlie' }
// ]

Pros:

• Works with objects
• Can specify which property to use for uniqueness
• Good performance

Cons:

• More complex syntax
• Keeps the last occurrence (can be modified to keep first)

Performance Comparison

For large arrays (10,000+ elements):

1. Set: ~1ms (fastest)
2. Map (for objects): ~2ms
3. Reduce: ~50ms
4. Filter: ~100ms (slowest)

Recommendation

• Use Set for primitive values (numbers, strings, booleans)
• Use Map for objects when you need to deduplicate by a specific property
• Use Filter or Reduce only when you need custom logic or are working with small arrays

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Note: All these methods create a new array. If you need to modify the original array in place, you’ll need a different approach.