使用普通JavaScript获取数组的重复项

function findDuplicates(arr) {
  var duplicates = [];
  
  for(var i = 0; i < arr.length; i++) {
    for(var j = i + 1; j < arr.length; j++) {
      if(arr[i] === arr[j] && duplicates.indexOf(arr[i]) === -1) {
        duplicates.push(arr[i]);
      }
    }
  }
  
  return duplicates;
}

var array = [1, 2, 3, 4, 3, 2, 1];
console.log(findDuplicates(array));

function findDuplicates(arr) {
  var duplicates = [];
  var count = {};
  
  for(var i = 0; i < arr.length; i++) {
    if(count[arr[i]] === undefined) {
      count[arr[i]] = 1;
    } else {
      if(count[arr[i]] === 1 && duplicates.indexOf(arr[i]) === -1) {
        duplicates.push(arr[i]);
      }
      count[arr[i]]++;
    }
  }
  
  return duplicates;
}

var array = [1, 2, 3, 4, 3, 2, 1];
console.log(findDuplicates(array));

function findDuplicates(arr) {
  var duplicates = [];
  var uniqueElements = new Set();
  
  for(var i = 0; i < arr.length; i++) {
    if(uniqueElements.has(arr[i]) && duplicates.indexOf(arr[i]) === -1) {
      duplicates.push(arr[i]);
    } else {
      uniqueElements.add(arr[i]);
    }
  }
  
  return duplicates;
}

var array = [1, 2, 3, 4, 3, 2, 1];
console.log(findDuplicates(array));