js数组去重

set

let arr = [1,1,2,2,3,4,5,5,6,7,7,8,9,9];
let newArr = [...new Set(arr)];
console.log(newArr);//[1, 2, 3, 4, 5, 6, 7, 8, 9]

filter

// 这种方法会有一个问题：[1,'1']会被当做相同元素，最终输入[1]
function unique(arr) {
  return arr.filter(function(item, index, arr) {
    //当前元素，在原始数组中的第一个索引==当前索引值，否则返回当前元素
    return arr.indexOf(item, 0) === index;
  });
}
let arr = [1,1,2,2,3,4,5,5,6,7,7,8,9,9];
let newArr = unique(arr);
console.log(newArr);//[1, 2, 3, 4, 5, 6, 7, 8, 9]

reduce

let arr =  [1,1,2,2,3,4,5,5,6,7,7,8,9,9];
let newArr = arr.reduce(function(ar,cur) {
  if(!ar.includes(cur)) {
    ar.push(cur)
  }
  return ar
},[]);
console.log(newArr);//[1, 2, 3, 4, 5, 6, 7, 8, 9]