开胃小菜--来给数组去个重？！

let arr = ['shanghai','beijing', 'hangzhou', 'guangzhou','nanjing','beijing','shanghai'];
function unrepeated(arr) {
  const res = [];
  arr.forEach(e => {
    !res.includes(e) && res.push(e);
  })
  return res;
}
unrepeated(arr);

let arr = ['shanghai','beijing', 'hangzhou', 'guangzhou','nanjing','beijing','shanghai'];
function unrepeated(arr) {
  return arr.filter((e,i) => arr.indexOf(e) === i);
}
unrepeated(arr);

let arr = ['shanghai','beijing', 'hangzhou', 'guangzhou','nanjing','beijing','shanghai'];
function unrepeated(arr) {
  const res = {};
  arr.forEach(e => {
    !res[e] && (res[e]=null);
  })
  return Object.keys(res);
}
unrepeated(arr);

let arr = ['shanghai','beijing', 'hangzhou', 'guangzhou','nanjing','beijing','shanghai'];
function unrepeated(arr) {
  const res = new Map();
  arr.forEach(e => {
    !res.has(e) && res.set(e);
    // 不加判断，直接覆盖也行
    // res.set(e);
  })
  return Array.from(res.keys());
}
unrepeated(arr);

let arr = ['shanghai','beijing', 'hangzhou', 'guangzhou','nanjing','beijing','shanghai'];
function unrepeated(arr) {
  return [...new Set(arr)]
}
unrepeated(arr);