count-duplicate.js

function countDuplicate(arr) {
  let map = new Map();
  arr.forEach(num => {
    if (map.get(num) > 0) {
      // set(key, value)
      map.set(num, map.get(num) + 1);
    } else {
      // set(key, value)
      map.set(num, 1);
    }
  });
  let count = 0;
  map.forEach((value, key) => {
    console.log('key:', key, ',value:', value);
    if (value > 1) {
      count += (value - 1);
    }
  })
  return count;
}

const res = countDuplicate([1, 4, 2, 4, 7, 1, 1, 9, 2, 3, 4, 1]);
console.log('result:', res);
/*
print:
key: 1 ,value: 4
key: 4 ,value: 3
key: 2 ,value: 2
key: 7 ,value: 1
key: 9 ,value: 1
key: 3 ,value: 1
result: 6
*/