How to flatten nested array of object using es6

问题:

I have this array of object, within it I have another array of object, how to get:

[
  { id: "5a60626f1d41c80c8d3f8a85" },
  { id: "5a6062661d41c80c8b2f0413" },
  { id: "5a60626f1d41c80c8d3f8a83" },
  { id: "5a60626f1d41c80c8d3f8a84" }
];

From:

[
  {
    id: 1,
    country: [
      {
        id: "5a60626f1d41c80c8d3f8a85"
      },
      {
        id: "5a6062661d41c80c8b2f0413"
      }
    ]
  },
  {
    id: 2,
    country: [
      {
        id: "5a60626f1d41c80c8d3f8a83"
      },
      {
        id: "5a60626f1d41c80c8d3f8a84"
      }
    ]
  }
];

without using a forEach and a temp variable?

When I did:

(data || []).map(o=>{
  return o.country.map(o2=>({id: o2.id}))
})

I got the same structure back.

回答1:

No need for any ES6 magic, you can just reduce the array by concatenating inner country arrays.

const data=[{id:1,country:[{id:"5a60626f1d41c80c8d3f8a85"},{id:"5a6062661d41c80c8b2f0413"}]},{id:2,country:[{id:"5a60626f1d41c80c8d3f8a83"},{id:"5a60626f1d41c80c8d3f8a84"}]}];

console.log(
  data.reduce(
    (arr, elem) => arr.concat(elem.country), []
  )
)

If you want an ES6 feature (other than an arrow function), use array spread instead of the concat method:

const data=[{id:1,country:[{id:"5a60626f1d41c80c8d3f8a85"},{id:"5a6062661d41c80c8b2f0413"}]},{id:2,country:[{id:"5a60626f1d41c80c8d3f8a83"},{id:"5a60626f1d41c80c8d3f8a84"}]}];

console.log(
  data.reduce(
    (arr, elem) => [...arr, ...elem.country], []
  )
)

Note: These suggestions would create a new array on each iteration.

For efficiency, you have to sacrifice some elegance:

const data=[{id:1,country:[{id:"5a60626f1d41c80c8d3f8a85"},{id:"5a6062661d41c80c8b2f0413"}]},{id:2,country:[{id:"5a60626f1d41c80c8d3f8a83"},{id:"5a60626f1d41c80c8d3f8a84"}]}];

console.log(
  data.reduce(
    (arr, elem) => {
      for (const c of elem.country) {
        arr.push(c);
      }
      return arr;
    }, []
  )
)

回答2:

const raw = [
  {
    id: 1,
    country: [
      {
        id: "5a60626f1d41c80c8d3f8a85"
      },
      {
        id: "5a6062661d41c80c8b2f0413"
      }
    ]
  },
  {
    id: 2,
    country: [
      {
        id: "5a60626f1d41c80c8d3f8a83"
      },
      {
        id: "5a60626f1d41c80c8d3f8a84"
      }
    ]
  }
];

const countryIds = raw
                    .map(x => x.country)
                    .reduce((acc, curr) => {
                      return [
                        ...acc, 
                        ...curr.map(x => x.id)
                      ];
                    }, []);
console.log(countryIds)

回答3:

This, works, just concat the nested arrays returned by your solution

let arr = [{    "id": 1,
    "country": [{
        "id": "5a60626f1d41c80c8d3f8a85",
      },
      {
        "id": "5a6062661d41c80c8b2f0413",
      }
    ]
  },
  {
    "id": 2,
    "country": [{
        "id": "5a60626f1d41c80c8d3f8a83",
      },
      {
        "id": "5a60626f1d41c80c8d3f8a84",
      }
    ]
  }
];

//If you want an array of country objects
console.log([].concat.apply(...(arr || []).map(o=> o.country)))

//If you can an array od country ids
console.log([].concat.apply(...(arr || []).map(o=> o.country.map(country => country.id))))

回答4:

Ayush Gupta's solution will work for this case. But I would like to provide other solution.

const arr = [
  {
    id: 1,
    country: [
      {
        id: '5a60626f1d41c80c8d3f8a85'
      },
      {
        id: '5a6062661d41c80c8b2f0413'
      }
    ]
  },
  {
    id: 2,
    country: [
      {
        id: '5a60626f1d41c80c8d3f8a83'
      },
      {
        id: '5a60626f1d41c80c8d3f8a84'
      }
    ]
  }
];

const ids = arr.reduce(
  (acc, {country}) => [
    ...acc,
    ...country.map(({id}) => ({
      id
    }))
  ],
  []
);
console.log(ids);

回答5:

For JSON string data, it can be done during parsing too :

var ids = [], json = '[{"id":1,"country":[{"id":"5a60626f1d41c80c8d3f8a85"},{"id":"5a6062661d41c80c8b2f0413"}]},{"id":2,"country":[{"id":"5a60626f1d41c80c8d3f8a83"},{"id":"5a60626f1d41c80c8d3f8a84"}]}]';

JSON.parse(json, (k, v) => v.big && ids.push(v));

console.log( ids );