JavaScript 数组对象去重的几种方法

var arr = [
  {id: 1,city: '南京'},
  {id: 2,city: '南京'}, 
  {id: 3,city: '杭州'}, 
  {id: 4,city: '广州'}
];

var obj = {};
var newArr = arr.reduce((prev,cur)=>{
  obj[cur.city] ? '':obj[cur.city] = true && prev.push(cur);
  return prev
},[]); //  [{id: 1,city: '南京'},{id: 3,city: '杭州'},{id: 4,city: '广州'}]  

for (var i = 0; i < arr.length - 1; i++) {
  for (var j = i + 1; j < arr.length; j++) {
    console.log(i,j)
    if (arr[i].city == arr[j].city) {
      arr.splice(j, 1); 
      j--; // 因为数组长度减小1，所以直接 j++ 会漏掉一个元素，所以要 j--
    }
  }
}
console.log(arr); //  [{id: 1,city: '南京'},{id: 3,city: '杭州'},{id: 4,city: '广州'}] 

var newArr = [];
arr.forEach((item) => {
  var check = newArr.every((b) => {
    return item.city !== b.city;
  })
  check ? newArr.push(item) : '';
})
console.log(newArr); //  [{id: 1,city: '南京'},{id: 3,city: '杭州'},{id: 4,city: '广州'}]  

let formatArr = () => {
  let map = new Map();
  for (let item of arr) {
    if (!map.has(item.city)) {
      map.set(item.city, item);
    }
  }
  return [...map.values()];
}
let newArr = formatArr(); //  [{id: 1,city: '南京'},{id: 3,city: '杭州'},{id: 4,city: '广州'}]