javascript extract objects from nested arrays using a list of ids

Question

I have a list of ids as reference, and I have an object which contains multiple objects that have array of objects.

I want to make an array of objects with corresponding ids in the list, the FASTEST way.

const data = {
  "items": {
    "item1": [{
        "id": "id1",
        "info": "info1"
      },
      {
        "id": "id2",
        "info": "info22"
      }
    ],
    "item20": [{
      "id": "id3",
      "info": "info5"
    }],
    "item5": [{
        "id": "id4",
        "info": "info6"
      },
      {
        "id": "id5",
        "info": "info7"
      }
    ]
  }
};

const keep = ['id4', 'id2'];
const results = [];
keep.forEach(function(val) {

  const match = Object.keys(data.items).map(item => {
    return data.items[item].find(obj => obj.id === val)
  });
  results.push(match)
})
console.log('final: ', results)

the current is not returning what i want. the expected result will be:

[
    {
      "id": "id2",
      "info": "info22"
    },
    {
        "id": "id4",
        "info": "info6"
    }
]

update:

How about in the case if the data is itself an array of objects, and we want to do the same for each one?

    const data = [{
      "otherStuff": "otherB",
      "items": {
        "item1": [{
            "id": "id1",
            "info": "info1"
          },
          {
            "id": "id2",
            "info": "info22"
          }
        ],
        "item20": [{
          "id": "id3",
          "info": "info5"
        }],
        "item5": [{
            "id": "id4",
            "info": "info6"
          },
          {
            "id": "id5",
            "info": "info7"
          }
        ]
      }
    }, {
      "otherStuff": "otherA",
      "items": {
        "item1": [{
            "id": "id1",
            "info": "info10000"
          },
          {
            "id": "id2",
            "info": "info220000"
          }
        ],
        "item20": [{
          "id": "id3",
          "info": "info5000"
        }],
        "item5": [{
            "id": "id4",
            "info": "info60000"
          },
          {
            "id": "id5",
            "info": "info7000"
          }
        ]
      }
    }];


const keep = ['id4', 'id2'];
const results = [];

    keep.forEach(function(val) {
      data.forEach(function(entry){
          Object.keys(entry.items).forEach(item => {
            var match = entry.items[item].find(obj => obj.id === val);
            if (match) {
              results.push(match)
            }
          });
      });
    })
    
    console.log(results)

and the output should be:

[
    {
        "otherStuff": "otherB",
        "items": [
                {
                      "id": "id2",
                      "info": "info22"
                },
                {
                    "id": "id4",
                    "info": "info6"
            }
        ]
    },
    {
        "otherStuff": "otherA",
        "items": [
                {
                      "id": "id2",
                      "info": "info220000"
                },
                {
                    "id": "id4",
                    "info": "info60000"
            }
        ]
    }
]

the result is not the same though.

Please visit the help center, take the tour to see what and How to Ask. Do some research, search for related topics on SO; if you get stuck, post a minimal reproducible example of your attempt, noting input and expected output. — mplungjan
– mplungjan, Commented Sep 4, 2018 at 14:37
@mplungjan fixed the typo in equal sign... i need an authoratitive answer to see what is the fastest way to do it. — teebeetee
– teebeetee, Commented Sep 4, 2018 at 14:43
@mplungjan No! "the current is not returning what i want" means the code is not working as expected which is one close reason on Code Review. — 301_Moved_Permanently
– 301_Moved_Permanently, Commented Sep 4, 2018 at 14:50

Jonas Wilms · Accepted Answer · 2018-09-04 14:47:39Z

3

If you use a Set for the kept ids, you save one O(n) traversal:

 const keep = new Set(['id4', 'id2']);

 const result = [];

 for(const items of Object.values(data.items))
   for(const item of items)
     if(keep.has(item.id))
       result.push(item);

answered Sep 4, 2018 at 14:47

Jonas Wilms

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2 Comments

phuzi Over a year ago

Not come across array.has can't find any documentation either.

phuzi Over a year ago

Ah, Set.prototype.has

Ankit Agarwal · Accepted Answer · 2018-09-04 14:36:03Z

1

You do not need to use map(). Simply use forEach() loop on the object keys which you want to keep and find the matching object so that it can be pushed into the result array.

const data = {
  "items": {
    "item1": [{
        "id": "id1",
        "info": "info1"
      },
      {
        "id": "id2",
        "info": "info22"
      }
    ],
    "item20": [{
      "id": "id3",
      "info": "info5"
    }],
    "item5": [{
        "id": "id4",
        "info": "info6"
      },
      {
        "id": "id5",
        "info": "info7"
      }
    ]
  }
};

const keep = ['id4', 'id2'];
const results = [];
keep.forEach(function(val) {
  Object.keys(data.items).forEach(item => {
    var match = data.items[item].find(obj => obj.id === val);
    if (match) {
      results.push(match)
    }
  });
})
console.log('final: ', results)

answered Sep 4, 2018 at 14:36

Ankit Agarwal

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3 Comments

teebeetee Over a year ago

is this the fastest way to do it? filter/find/reduce/ can't they be faster?

Ankit Agarwal Over a year ago

@storis you have array of objects inside the object which will surely have O(n2) complexity

teebeetee Over a year ago

i updated the question with an annex, could you please have a look at it? how can I achieve the similar results in this case?

brk · Accepted Answer · 2018-09-04 14:45:34Z

items is an object & item1,item20 are are keys.So you can do Object.values and apply reduce function on it to get a single array which is consist of all the objects. No iterate the keep array and filter out the required element from the single array.

const data = {
  "items": {
    "item1": [{
        "id": "id1",
        "info": "info1"
      },
      {
        "id": "id2",
        "info": "info22"
      }
    ],
    "item20": [{
      "id": "id3",
      "info": "info5"
    }],
    "item5": [{
        "id": "id4",
        "info": "info6"
      },
      {
        "id": "id5",
        "info": "info7"
      }
    ]
  }
};



const keep = ['id4', 'id2'];
let getAllObjectValues = Object.values(data.items).reduce(function(acc, curr) {
  curr.forEach((elem) => {
    acc.push(elem)
  })
  return acc;
}, []);

let k = keep.map(function(item) {
  return getAllObjectValues.filter(function(val) {
    return item === val.id
  })[0]

})
console.log(k)

Chris Cousins · Accepted Answer · 2018-09-04 14:32:17Z

0

You've used an assignment operator rather than an equivalence operator in your map function, change to:

const keep = ['id4', 'id2'];
const results = [];
keep.forEach(function(val){
    const match = Object.keys(data.items).map(item => {
        return data.items[item].find(obj => obj.id === val)
    });
    results.push(match)
})
console.log('final: ', results)

answered Sep 4, 2018 at 14:32

Chris Cousins

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1 Comment

mplungjan Over a year ago

No longer a typo-type question but still off topic

phuzi · Accepted Answer · 2018-09-04 14:53:26Z

Using a combination of reduce and filter you can iterate each sub-array checking whether the value should be kept.

const data = {
  "items": {
    "item1": [
      {
        "id": "id1",
        "info": "info1"
      },
      {
        "id": "id2",
        "info": "info22"
      }
    ],
    "item20": [
      {
        "id": "id3",
        "info": "info5"
      }
    ],
    "item5": [
      {
        "id": "id4",
        "info": "info6"
      },
      {
        "id": "id5",
        "info": "info7"
      }
    ]
  }
};

const keep = ['id4', 'id2'];
const filter = el => {
  return keep.indexOf(el.id) >= 0;
};

const reducer = (accumulator, currentValue) => { 
  return accumulator = accumulator.concat(data.items[currentValue].filter(filter));
};

let results = Object.keys(data.items).reduce(reducer, []);

console.log('final: ', results);

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