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How do I write select distinct country, state from myObject in javascript.

Array.from(new Set(myObject.map(item => item.country))) would return distinct countries.. How do I add countries, states and other columns in map???

Input data:

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

To get unique countries, I would write Array.from(new Set(myObject.map(item => item.country))) The result would be ['USA', 'Canada']

What should I write if want the result to be

[
  {
    country: 'USA',
    state: 'Missouri',
  },
  {
    country: 'Canada',
    state: 'Alberta',
  },
]

This contains unique country and state binations

How do I write select distinct country, state from myObject in javascript.

Array.from(new Set(myObject.map(item => item.country))) would return distinct countries.. How do I add countries, states and other columns in map???

Input data:

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

To get unique countries, I would write Array.from(new Set(myObject.map(item => item.country))) The result would be ['USA', 'Canada']

What should I write if want the result to be

[
  {
    country: 'USA',
    state: 'Missouri',
  },
  {
    country: 'Canada',
    state: 'Alberta',
  },
]

This contains unique country and state binations

• javascript
• angular
• typescript

Share Improve this question Follow edited Oct 14, 2020 at 13:18 jcalz 332k2929 gold badges442442 silver badges442442 bronze badges asked Oct 14, 2020 at 1:42 Nikhila Reddy CNikhila Reddy C 5911 gold badge22 silver badges88 bronze badges 4

• could you provide an example of the input data and the expected result? not sure if this is what you're looking for stackoverflow./questions/56633869/… – diedu Commented Oct 14, 2020 at 1:53
• I edited the question with sample data and expected result. – Nikhila Reddy C Commented Oct 14, 2020 at 2:21
• @diedu I'm not sure why you think this question is not related to TypeScript? – jcalz Commented Oct 14, 2020 at 2:23
• I updated my answer to use your example. If it does not address your use case, please elaborate. – jcalz Commented Oct 14, 2020 at 13:21

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3 Answers 3

Sorted by: Reset to default 2

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./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 es 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 es 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

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