findOdd.js

function findOdd(A) {
  const freq = {}; //create an empty obj
  for (const num of A) { //iterate through the arr
    freq[num] = (freq[num] || 0) + 1; //push the number and the times it appears on the arr as [key,val] pair
  }
  return +Object.keys(freq).find((num) => freq[num] % 2 == 1); //return the key that has a value with an odd number
}

// function findOdd(A) {
//   //happy coding!
//   let found = {};
//   for (let i = 0; i < A.length; i++) {
//     let keys = A[i].toString();
//     found[keys] = ++found[A[i]] || 1;
//   }

//   return Number(Object.keys(found).find((n) => found[n] % 2 != 0));
// }

// function findOdd(A) {
//   const appearances = {};
//   A.forEach((val) => {
//     appearances[val] = (appearances[val] || 0) + 1;
//   });
//   const oddNumbers = Object.keys(appearances).filter(
//     (key) => appearances[key] % 2 != 0
//   );
//   return oddNumbers.length > 0 ? oddNumbers[0] : NaN;
// }

console.log(findOdd([20, 1, -1, 2, -2, 3, 3, 5, 5, 1, 2, 4, 20, 4, -1, -2, 5]));