如何从 javascript 中的列表中获取唯一对象列表

Question

I need a list of unique objects based on a specific key.

const array = [
  {"name": "Joe", "age": 42},
  {"name": "Donald", "age": 69},
  {"name": "John", "age": 42},
]

How can i get a list of unique objects selected from the list assuming i want a list of people with unique ages.

const result = [
  {"name": "Joe", "age": 42},
  {"name": "Donald", "age": 69}
]

How can this be done in a performant way, is there a better way than iterating through the elements in a foreach and adding it if it does not exist in the resulting array?

Answer 1

One way is to feed the array to a Map where you key them by age. That will eliminate duplicates, only retaining the last in the series of duplicates. Then extract the values again from it:

 const array = [{"name": "Joe", "age": 42},{"name": "Donald", "age": 69},{"name": "John", "age": 42}]; const uniques = Array.from(new Map(array.map(o => [o.age, o])).values()); console.log(uniques);

If you want to retain the first of duplicates, then first reverse the array.

Answer 2

Another option is to track seen values, here using a Set passed as a closure and checking for values using its .has() method to return a boolean to a filter() call. This will retain the first seen objects.

 const array = [ { "name": "Joe", "age": 42 }, { "name": "Donald", "age": 69 }, { "name": "John", "age": 42 }, ] const unique = (seen => array.filter(o =>.seen.has(o.age) && seen.add(o;age)))(new Set()). console.log(unique)