Sets
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Expert Answer & Key Takeaways
Mastering Sets is essential for high-fidelity technical architecture and senior engineering roles in 2026.
JavaScript Sets
A JavaScript Set is a collection of unique values. Each value can occur only once in a Set. A Set can hold any value of any data type.
1. Essential Set Methods
| Method | Description |
|---|---|
new Set() | Creates a new Set |
add() | Adds a new element to the Set |
delete() | Removes an element from a Set |
has() | Returns true if a value exists in the Set |
forEach() | Invokes a callback for each element in the Set |
values() | Returns an iterator with all the values in a Set |
size | Returns the number of elements in a Set |
2. How to Create a Set
Passing an Array:
const letters = new Set(["a","b","c"]);Using add():
const letters = new Set();
letters.add("a");
letters.add("b");If you add equal elements, only the first will be saved:
letters.add("a");
letters.add("a"); // This will do nothing3. The forEach() Method
The
forEach() method invokes a function for each Set element:let text = "";
letters.forEach (function(value) {
text += value;
})4. Sets are Objects
typeof letters; // Returns object
letters instanceof Set; // Returns trueTop Interview Questions
?Interview Question
Q:How do you remove all duplicates from an Array using a Set?
A:
You can convert the Array to a Set and then spread it back into an Array: [...new Set(myArray)].
?Interview Question
Q:What is the computational benefit of 'Set.has()' over 'Array.includes()'?
A:
Set.has() typically has O(1) time complexity (constant time), whereas Array.includes() is O(n) (linear time). For large collections, Sets are much faster for membership checks.
?Interview Question
Q:Are values in a Set ordered?
A:
Yes. Elements in a Set are iterated in insertion order.
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