Unique values and key-value collections.
✍️ 3 exercises + code challenge in this lessonA Set stores unique values. Adding a duplicate does nothing.
add(), has(), delete(), sizeA common trick: remove duplicates from an array with [...new Set(array)].
A Map stores key-value pairs where keys can be any type, and it remembers insertion order.
set(), get(), has(), delete(), sizeMastering JavaScript Sets & Maps helps you write clean and reliable JavaScript code. Follow standard conventions, test edge cases, and keep logic simple. Try running the code example below and complete the exercises to build confidence.
<pre id="out"></pre>
<script>
const log = (...a) => document.getElementById("out").textContent += a.join(" ") + "\n";
const tags = new Set(["js", "css", "js", "html", "css"]);
tags.add("python");
log("Set:", [...tags].join(", "), "| size:", tags.size);
log("Has css?", tags.has("css"));
const unique = [...new Set([3, 1, 3, 2, 1])];
log("Unique:", unique);
const prices = new Map();
prices.set("Keyboard", 2500).set("Mouse", 1200);
log("Mouse costs", prices.get("Mouse"));
for (const [item, price] of prices) log(item, "=", price);
</script>Check what you learned in “JavaScript Sets & Maps”.
Remove duplicates from an array.
const unique = [...new (list)];
Which Map method reads a value by its key?
Log the number of unique values in [1, 2, 2, 3, 3, 3].
Free sign-in required · Google login available
Stuck on something?
Ask the community in the CodesInTech forum.
Need a developer?
Learning on your own is great — and when you need professional help, our team builds custom software, websites and mobile apps for clients worldwide.
Explore Our Services