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How to filter array of object inside the array of the object type in JavaScript?

January 19, 2022

I’m facing one issue with one of the API responses,
I have a response something like the below one.

[
  {type: "StateCountry", state: "AL", countries: [{type: "County", countyName: "US"}, {type: "County", countyName: "US"}, {type: "County", countyName: "US"}]},
  {type: "StateCountry", state: "AL", countries: [{type: "County", countyName: "German"}, {type: "County", countyName: "German"}, {type: "County", countyName: "German"}]},
  {type: "StateCountry", state: "AL", countries: [{type: "County", countyName: "Japan"}, {type: "County", countyName: "German"}, {type: "County", countyName: "German"}]},
]

it looks like an array of objects inside that have countries also the arrays of the object type. I need all the countyName in a single array for all the array of elements. like the below one type,

newNames = [US, German, Japan];

I tried something like this. but I couldn’t able to get the output. can you please help me to do this? thank you all.

let newNames = this.selectedStateList.filter(item => item.countries.forEach(item => item.countries)).map(ele => ele.countries.forEach(item => item.countries))

>Solution :

You can look through each countries object and add your data in a Set to get unique country.

const data = [ {type: "StateCountry", state: "AL", countries: [{type: "County", countyName: "US"}, {type: "County", countyName: "US"}, {type: "County", countyName: "US"}]}, {type: "StateCountry", state: "AL", countries: [{type: "County", countyName: "German"}, {type: "County", countyName: "German"}, {type: "County", countyName: "German"}]}, {type: "StateCountry", state: "AL", countries: [{type: "County", countyName: "Japan"}, {type: "County", countyName: "German"}, {type: "County", countyName: "German"}]}, ],
      result = Array.from(data.reduce((r, {countries}) => {
        countries.forEach(({countyName}) => r.add(countyName));
        return r;
      }, new Set()));
console.log(result);