A Set is a collection of values where each value must be unique. Internally, most sets use a hash table to ensure that checking for the existence of an item takes constant time.

Think of a Set as a Guest List for a private party:

1. When someone arrives, you check if their name is on the list.
2. If they are already there, you don't let a second "Duplicate" of them in.
3. You can quickly tell if someone is at the party without looking at everyone inside.

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Core Properties

• No Duplicates: Automatically handles redundant data.
• O(1) Operations: Adding, Removing, and Checking (has) are all constant time.
• Set Operations: Support for Mathematical operations like Union, Intersection, and Difference.

Set vs. Array

• Array: Keeps items in order, allowed to have duplicates, searching takes $O(n)$ time.
• Set: Doesn't guarantee order (usually), strictly unique, searching takes $O(1)$ time.

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javascript Standard
/**
 * Using the native JavaScript Set object
 */
const guestList = new Set(['Alice', 'Bob']);

// 1. Adding (O(1))
guestList.add('Charlie');
guestList.add('Alice'); // Does nothing, Alice is already there

// 2. Checking Existence (O(1))
if (guestList.has('Alice')) {
  console.log("Alice is here!");
}

// 3. Removing
guestList.delete('Bob');

// 4. Converting back to Array (Useful for removing duplicates)
const uniqueItems = [...guestList];

/**
 * Finding Intersection of two arrays
 */
function intersection(arr1, arr2) {
  const set1 = new Set(arr1);
  // Filter arr2 for items that exist in set1
  return arr2.filter(item => set1.has(item));
}

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Use Cases

• Removing Duplicates: The fastest way to deduplicate an array is [...new Set(arr)].
• Existential Questions: "Have we seen this number before?" $\rightarrow$ Use a Set.
• Cycle Detection: Storing visited nodes in a graph or linked list.

Common Questions

• Intersection of Two Arrays: Use a set for the smaller array.
• Happy Number: Use a set to detect if you've entered a loop of numbers.
• Contains Duplicate: Check if set.size !== array.length.
• Longest Consecutive Sequence: A high-tier problem solved using a Set in $O(n)$.