Sets — Complete Guide
Sets — Complete Guide: free step-by-step lesson with examples, common mistakes, and interview tips — part of JavaScript Tutorial on Toolliyo Academy.
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JavaScript Tutorial · Lesson 27 of 100
Sets
Basics ✓ → Objects & data → Async & DOM → Advanced → Tools → Projects
Intermediate · 2 — Built-in types & objects · ~6 min · JS Objects & Collections
What is this?
A Set stores unique values — no duplicates. Great for tags, visited IDs, or deduplicating a list.
Why should you care?
Checking uniqueness in an array is slow; Set.has is fast.
See it live — copy this example
Paste into an HTML file or the browser console (F12). Use Run below when the live editor is available.
const ids = new Set([1, 2, 2, 3]);
ids.add(4);
console.log(ids.size);
console.log(ids.has(2));Run Example »
Edit the code below and click Run to see the result in Toolliyo’s live editor.
What happened?
- Duplicate 2 was ignored.
- add inserts if not present.
- size counts unique items.
Practice next
- Create Set from duplicate array and log size.
- add, has, and delete values.
- Convert back with [...new Set(arr)].
- Store visited page paths in a Set and check has before logging analytics twice.
- Clear all entries with ids.clear().
Remember
new Set(iterable) Unique values only has/add/delete/size
ScriptVerse tag picker
Product tags use a Set so "sale" and "Sale" duplicates are normalized and blocked at add time.
Outcome: Sets give O(1) uniqueness checks for filters and analytics deduplication.
Interview prep for this lesson
Practice these questions aloud after reading—each links to a full structured answer.
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