如何从angular 6中的数组列表中查找重复项？

const arr = [1, 2, 3, 4, 1, 5, 2];
const duplicates = arr.filter((item, index) => arr.indexOf(item) !== index);
console.log(duplicates); // [1, 2]

const arr = [1, 2, 3, 4, 1, 5, 2];
const count = arr.reduce((acc, curr) => {
  acc[curr] = (acc[curr] || 0) + 1;
  return acc;
}, {});

const duplicates = Object.keys(count).filter((item) => count[item] > 1);
console.log(duplicates); // ["1", "2"]

import { groupBy } from 'lodash';

const arr = [1, 2, 3, 4, 1, 5, 2];
const grouped = groupBy(arr);
const duplicates = Object.keys(grouped).filter((item) => grouped[item].length > 1);
console.log(duplicates); // ["1", "2"]

import { of } from 'rxjs';
import { groupBy, mergeMap, toArray } from 'rxjs/operators';

const arr = [1, 2, 3, 4, 1, 5, 2];
const duplicates = of(...arr)
  .pipe(
    groupBy((item) => item),
    mergeMap((group) => group.pipe(toArray())),
    toArray(),
    filter((group) => group.length > 1),
  )
  .subscribe((result) => console.log(result)); // [[1, 1], [2, 2]]