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

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))

7 months ago · Juan Pablo Isaza

3 answers

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You need .maps, not .forEach - .maptransforms each element of an array, butforEach` does not return anything.

After getting an array of arrays of country names, you can flatten with .flat() - or you can put them together and use flatMap instead.

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

const output = input.flatMap(
  obj => obj.countries.map(
    country => country.countyName
  )
);
console.log(output);

7 months ago · Juan Pablo Isaza Report

0

Would something like this work?

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


const result = arr.reduce((acc,val)=>{
acc.push(val.countries[0].countyName)
return acc
},[] )

console.log(result)

7 months ago · Juan Pablo Isaza Report

Answer question

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Accepted Answer · 2022-07-05T05:08:05.982Z

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);