← ⚡ JavaScript World

⚡ No Duplicates Allowed

★★★★☆ · worth 200 points

You can practise without a player, but pick a player to score points!

📖 The mission

Today you unlock

Loops inspect each item

Many levels use a loop: start with a simple value, visit each item, update the answer, then return it at the end.

💡 Hint ladder

Keep a result array; only push items you have not seen yet.
items.includes(x) or a Set can check "seen before".
One-liner: [...new Set(items)]

🧪 Coach checks

◯ removes repeats
◯ numbers too
◯ already unique
◯ empty stays empty

Live coach

Getting ready

☆☆☆

💬 includes() asks "have I seen this before?" — perfect for skipping repeats.

const seen = ["a", "b"];

console.log(seen.includes("a"));  // true
console.log(seen.includes("z"));  // false

📟 It printed:

📟 Output