Typescript get distinct values from an object

Below is a solution using reduce;

const myObject = [
  { country: 'USA', state: 'Missouri', county: 'County1' },
  { country: 'USA', state: 'Missouri', county: 'County2' },
  { country: 'Canada', state: 'Alberta', county: 'County3' },
];

const unique = myObject.reduce((prev, {country, state}) => 
  prev.some(x => x.country === country && x.state === state )? prev: [...prev, {country, state} ], [])

   console.log(unique )

Base on referenced answer you can use Map and spread operator as below

https://stackoverflow.com/a/63274702/4964569

const myObject = [
  { country: 'USA', state: 'Missouri', county: 'County1' },
  { country: 'USA', state: 'Missouri', county: 'County2' },
  { country: 'Canada', state: 'Alberta', county: 'County3' },
];

let data = new Map();

for (let obj of myObject) {
  delete obj.county;
  data.set(obj.country, obj);
}

let unique = [...data.values()];

console.log(unique);

The following code should take an array of objects and a list of keys of those objects, and return an array of objects representing the distinct values for that set of keys. I assume that the type of properties at those keys will only be string, number, or boolean.

function distinct<T extends Record<K, string | number | boolean>,
  K extends keyof T>(arr: T[], ...keys: K[]): Pick<T, K>[] {
  const key = (obj: T) => JSON.stringify(keys.map(k => obj[k]));
  const val = (obj: T) => keys.reduce((a, k) => (a[k] = obj[k], a), {} as Pick<T, K>);
  const dict = arr.reduce((a, t) => (a[key(t)] = val(t), a), {} as { [k: string]: Pick<T, K> })
  return Object.values(dict);
}

The idea is to take each object in the array, serialize the tuple of its properties at the keys in question with JSON.stringify(), and use this serialized string as a dictionary key. The value we put at that key is an object consisting of just the values at those properties. By using a dictionary, we guarantee that only one object will appear for each distinct set of properties at the keys we care about. Then we turn the values of this dictionary into an array.

If I test it on your myObject example, this is what comes out:

const result = distinct(myObject, "country", "state");
console.log(result);
/*  [{ "country": "USA", "state": "Missouri" }, { "country": "Canada", "state": "Alberta" }] */

which is what you wanted. Let's also test it on some type and array that I made up now:

interface MyObject {
  country: string,
  state: string,
  age: number,
  name: string
}
const arr: MyObject[] = [
  { name: "Alice", age: 35, country: "USA", state: "MA" },
  { name: "Bob", age: 40, country: "USA", state: "MI" },
  { name: "Carmen", age: 35, country: "Mexico", state: "BC" },
  { name: "Danilo", age: 35, country: "Mexico", state: "MI" }
]

So we have an array of MyObject. Let's get the distinct country values:

const distinctCountries = distinct(arr, "country"); // Array<{country: string}>
console.log(distinctCountries); // [{ "country": "USA" }, { "country": "Mexico" }] 
console.log(distinctCountries.map(x => x.country)); // USA, Mexico

Notice how what comes out is an array of {country: string} values. You can use map() to turn that into just an array of strings. Let's get the distinct country and state values:

const distinctCountriesAndStates = distinct(arr, "country", "state");
console.log(distinctCountriesAndStates);
/* [{ "country": "USA", "state": "MA" }, { "country": "USA", "state": "MI" }, 
{ "country": "Mexico", "state": "BC" }, { "country": "Mexico", "state": "MI" }] */

Here it's an array of {country: string, state: string} objects. Not sure how you want to represent those, but you can use map() to massage them as you see fit. Finally let's get the distinct country and age values:

const distinctAgesAndCountries = distinct(arr, "country", "age");
console.log(distinctAgesAndCountries);
/* [{ "country": "USA", "age": 35 }, { "country": "USA", "age": 40 },
 { "country": "Mexico", "age": 35 }] */

That's an array of {country: string, age: number} objects.

Anyway, hope that gives you some direction.

Playground link to code