How to find unique values in an array?

const arr = [1, 2, 2, 3, 3, 4];
 
// Set — most common way
const unique1 = [...new Set(arr)]; // [1, 2, 3, 4]
 
// filter with indexOf
const unique2 = arr.filter((item, index) => arr.indexOf(item) === index); // [1, 2, 3, 4]
 
// Both work, but Set is faster!

const colors = ['red', 'blue', 'red', 'green', 'blue'];
 
// Create Set (unique values) and back to array
const unique = [...new Set(colors)];
console.log(unique); // ['red', 'blue', 'green']
 
// Works with any values
const mixed = [1, '1', 1, true, true, false];
console.log([...new Set(mixed)]); // [1, '1', true, false]

const numbers = [1, 2, 2, 3, 3, 4];
 
const unique = numbers.filter((item, index) => {
  return numbers.indexOf(item) === index;
});
 
console.log(unique); // [1, 2, 3, 4]
 
// How it works:
// For 1: indexOf(1) = 0, index = 0 → 0 === 0 ✅
// For 2: indexOf(2) = 1, index = 1 → 1 === 1 ✅
// For 2: indexOf(2) = 1, index = 2 → 1 !== 2 ❌ (skip)

const arr = [1, 2, 2, 3, 3, 4];
 
const unique = arr.reduce((acc, current) => {
  if (!acc.includes(current)) {
    acc.push(current);
  }
  return acc;
}, []);
 
console.log(unique); // [1, 2, 3, 4]

// ✅ Simple and fast
const arr = [1, 2, 2, 3, 3, 4];
const unique = [...new Set(arr)];

// ✅ Works everywhere, even old browsers
const arr = [1, 2, 2, 3, 3, 4];
const unique = arr.filter((item, index) => arr.indexOf(item) === index);

// ✅ When you need special accumulation logic
const arr = [1, 2, 2, 3, 3, 4];
const unique = arr.reduce((acc, current) => {
  // Custom logic...
  return acc;
}, []);

// ❌ Set won't help with objects
const arr = [{id: 1}, {id: 1}, {id: 2}];
const unique = [...new Set(arr)]; // Doesn't work!
 
// ✅ Need custom logic
const uniqueObjects = arr.filter((item, index, self) => 
  index === self.findIndex(obj => obj.id === item.id)
);

// ✅ Works fine
const arr = [1, null, undefined, 1, null];
console.log([...new Set(arr)]); // [1, null, undefined]

// ❌ Mistake
const arr = [[1, 2], [1, 2], [3, 4]];
const unique = [...new Set(arr)]; // Won't remove duplicates!
 
// ✅ Correct — convert to strings
const uniqueArrays = [...new Set(arr.map(JSON.stringify))].map(JSON.parse);

// ❌ Slow for large arrays
const arr = [/* large array */];
arr.filter(item => arr.indexOf(item) === arr.lastIndexOf(item)); // Very slow!
 
// ✅ Fast
const unique = [...new Set(arr)];