duplicates-4.js

const nums = [2, 6, 3, 10]
const nums2 = [2, 4, 1, 2]

const duplicates4 = arr => {
  
  // Start with an empty Object as a 
  // record keeper
  const obj = {}

  for (let i = 0; i < arr.length; i++) {
    const num = arr[i]

    // If we have already seen this number 
    // before it must be a duplicate!
    if(obj[num]) {
      return true
    }
    
    // We haven't seen it before so let's
    // start tracking it
    else {
      obj[num] = true
    }
  }

  // We looked at everything and no matches! 
  // There are no duplicates
  return false
}

console.log(duplicates3(nums))  // false
console.log(duplicates3(nums2)) // true