Set in JavaScript (original) (raw)

Last Updated : 05 Mar, 2025

A Set in JavaScript is used to store a unique collection of items, meaning no duplicates are allowed.

• Sets internally use a hash table which makes search, insert and delete operations faster than arrays. Please note that a hash table data structure allows these operations to be performed on average in constant time.
• Set maintains the insertion of items. When we access items, we get them in the same order as inserted.
• Only Unique Keys are allowed, if we insert the same key with a different value, it overwrites the previous one.
• A set can be created either empty or by providing an iterable like array or string. JavaScript `

// using an array let s1 = new Set([10, 30, 30, 40, 40]); console.log(s1); let s2 = new Set(["gfg", "gfg", "geeks"]); console.log(s2);

// using string let s3 = new Set("fooooooood"); console.log(s3);

// an empty set let s4 = new Set(); console.log(s4);

`

Output

Set(3) { 10, 30, 40 } Set(2) { 'gfg', 'geeks' } Set(3) { 'f', 'o', 'd' } Set(0) {}

How Set Internally Works in JavaScript

In JavaScript, hash tables are generally used to implement the property of unique values in the **Set data structure. The core property of a **Set is that it stores only unique values, and any duplicate values are automatically ignored.

**Process of Storing Data in a Hash Table

1. **Iteration Over Values: The first step involves iterating over all the values passed in an array, which is then passed to the **Set constructor function. This iterator ensures that each value is processed individually.
2. **Hashing with a Hash Function: A hash function, denoted as H(), is introduced. This function takes the value and converts it into a numerical index. The hash function’s purpose is to assign each unique value a specific index in the table.
3. **Mapping Values to Hash Table: In this step, the respective indices for each value are mapped onto the hash table. For example, if the value 1 produces index 5 from the hash function, the value 1 will be stored at index 5 in memory.
4. **Handling Duplicates: When a duplicate value is attempted to be added to the **Set, the hash function produces the same index for that value. The algorithm then checks whether that index already contains a value. If the index is already occupied, the new value is discarded, and the previous value is retained, ensuring the uniqueness of values in the **Set.

**Value Retrieval from the Hash Table

In a hash table, each unique value is stored at a separate index. To retrieve a value from the hash table, the index must be known beforehand. This allows **constant-time (O(1)) retrieval since the index is directly used to access the value.

Hash tables are considered optimized and fast for storing data, ensuring efficient management of the **Set data structure’s functionality, such as maintaining unique values and enabling quick lookups.

Key Characteristics of Sets

• **Unique Values: Sets only contain unique elements, automatically removing duplicates.
• **Ordered: Sets maintain the order of elements as they are inserted.
• **Iterable: You can iterate through Sets using loops like for…of or forEach.
• **No Indexing: Sets don’t support indexing; you must iterate to access elements.

**Methods of Set in JavaScript

• **Set.add() : adds the new element with a specified _value at the end of the Set object.
• **Set.delete() : deletes an element with the specified _value from the Set object.
• **Set.clear(): removes all the element from the set.
• **Set.entries() : returns an iterator object which contains an array having the entries of the set, in the insertion order.
• **Set.has() : returns true if the specified _value is present in the Set object.
• **Set.values() : returns all the values from the Set in the same insertion order.
• **Set.keys(): also returns all the values from the Set in the insertion order.
• **Set.forEach() : executes the given _function once for every element in the Set, in the insertion order.
• **Set.prototype@@iterator : returns a Set iterator function which is _values() function by default.
• **has(value):Checks if a value exists in the Set.

DSA Problems On Set In JavaScript.

• Union of Two Arrays
• Intersection of Two Arrays
• Distinct Elements in an Array
• Check for Disjoint Arrays or Sets
• Check if an Array is Subset of Other
• Duplicate within K Distance in an Array
• 2 Sum – Check for Pair with Target Sum
• Only Repeating Element From 1 To n-1
• Longest Consecutive Sequence
• Smallest Subarray with k Distinct Numbers