javascript - 如何在 Javascript 对象数组中查找特定键的所有唯一值？

Question

我有一个 javascript 对象数组，我正在尝试获取每个对象中特定属性的所有唯一值的数组。我尝试使用 reduce() 执行此操作，下面是我的示例代码，但它会导致错误“无法读取未定义的属性(读取‘includes’)”，即使我提供了一个空数组的初始值。预期的结果是一个数组

['New York', 'San Francisco', 'Chicago', 'Los Angeles']

最终目标是创建一个条形图，其中 X 轴为城市，Y 轴为每个城市的计算平均工资，因此我需要唯一的城市列表。有没有办法避免这个错误，或者有更好的方法来完全做到这一点？

const employees= [
 {id: 0, city: 'New York', wagePerHour: '15'}, 
 {id: 1, city: 'San Francisco', wagePerHour: '18'}, 
 {id: 2, city: 'New York', wagePerHour: '16'}, 
 {id: 3, city: 'Chicago', wagePerHour: '14'}, 
 {id: 4, city: 'Chicago', wagePerHour: '12'}, 
 {id: 5, city: 'San Francisco', wagePerHour: '15'}, 
 {id: 6, city: 'New York', wagePerHour: '18'}, 
 {id: 7, city: 'Los Angeles', wagePerHour: '10'}
];
const cities = employees.reduce((foundValues, nextEmployee) => {
 if(! foundValues.includes(nextEmployee.city)) {
  foundValues.push(nextEmployee.city);
 }
}, []);

Answer 1

一种更简单的方法是提取所有城市名称并使用 Set 函数构建一个唯一数组:

const employees = [{ id: 0, city: 'New York', wagePerHour: '15' }, { id: 1, city: 'San Francisco', wagePerHour: '18' }, { id: 2, city: 'New York', wagePerHour: '16' }, { id: 3, city: 'Chicago', wagePerHour: '14' }, {  id: 4, city: 'Chicago', wagePerHour: '12' }, { id: 5, city: 'San Francisco', wagePerHour: '15' }, {  id: 6, city: 'New York', wagePerHour: '18' }, { id: 7, city: 'Los Angeles', wagePerHour: '10' }]

let cities = [... new Set(employees.map(x=>x.city))];

console.log(cities);