Javascript: Merge Two Arrays of Objects, Only If Not Duplicate (Based on Specified Object Key)

user_2111901 0
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Background

Say I have an initial array of objects:

var initialData = [
    {
        'ID': 1,
        'FirstName': 'Sally'
    },
    {
        'ID': 2,
        'FirstName': 'Jim'
    },
    {
        'ID': 3,
        'FirstName': 'Bob'
    }
];

I then get new data (another array of objects):

var newData = [
    {
        'ID': 2,
        'FirstName': 'Jim'
    },
    {
        'ID': 4,
        'FirstName': 'Tom'
    },
    {
        'ID': 5,
        'FirstName': 'George'
    }
];

Goal

I want to merge the new data into initial data. However, I don't want to overwrite any objects in the initial data array. I just want to add in objects that weren't already there.

I know the objects are duplicates based on their 'ID' key.

What I've Tried

I know I can do this by looping through the new data, checking to see if it exists in the initial data, and if not, pushing into initial data.

for ( var i = 0, l = newData.length; i < l; i++  ) {

    if ( ! key_exists( newData[i].key, initialData ) ) {  // key_exists() is a function that uses .filter() to test.

        initialData.push( newData[i] );

    }


}

I'm concerned about performance, though. I know there are lots of new ES6 ways of manipulating arrays, so I'm hoping someone has a better idea.

Question

What is the best way (best as in best performance) of merging the new data into the initial data, while ignoring duplicates in new data?

arrays javascript
3 Answers

You can create a set of IDs from initialData and this will make "check if ID is already in initial data" faster - O(1):

var initialData = [{
    'ID': 1,
    'FirstName': 'Sally'
  },
  {
    'ID': 2,
    'FirstName': 'Jim'
  },
  {
    'ID': 3,
    'FirstName': 'Bob'
  }
];

var newData = [{
    'ID': 2,
    'FirstName': 'Jim'
  },
  {
    'ID': 4,
    'FirstName': 'Tom'
  },
  {
    'ID': 5,
    'FirstName': 'George'
  }
];

var ids = new Set(initialData.map(d => d.ID));
var merged = [...initialData, ...newData.filter(d => !ids.has(d.ID))];

console.log(merged);

The final runtime of this approach is O(n + m).

If you want to be slightly more efficient, you can consider looping through newData and pushing any new elements to the final result array manually (instead of using filter and the spread operator).

user_18226980

Actually, if you are interested on performance, you could think on changing your initialData structure to something like this:

var initialData = {
    "1": {'FirstName': 'Sally'},
    "2": {'FirstName': 'Jim'},
    "3": {'FirstName': 'Bob'}
};

In other words, we use the IDs as the keys of an object, this will give you O(1) on access the data, and O(1) in the exists test. You can get this structure using the next approach with reduce():

var initialData = [
    {'ID': 1, 'FirstName': 'Sally'},
    {'ID': 2, 'FirstName': 'Jim'},
    {'ID': 3, 'FirstName': 'Bob'}
];

let newInitialData = initialData.reduce((res, {ID, FirstName}) =>
{
    res[ID] = {FirstName : FirstName};
    return res;
}, {});

console.log(newInitialData);

Using this new structure, you can make a O(n) algorithm to insert the new data that is not already there:

var initialData = {
    "1": {'FirstName': 'Sally'},
    "2": {'FirstName': 'Jim'},
    "3": {'FirstName': 'Bob'}
};

var newData = [
    {'ID': 2, 'FirstName': 'Jim'},
    {'ID': 4, 'FirstName': 'Tom'},
    {'ID': 5, 'FirstName': 'George'}
];

newData.forEach(({ID, FirstName}) =>
{
    initialData[ID] = initialData[ID] || {FirstName: FirstName};
});

console.log(initialData);

user_103664950

I think the alternative solution proposed by @slider in the accepted answer, would be something like this:

const oldData = [
  { id: 1, name: 'John' },
  { id: 2, name: 'Jane' },
  { id: 3, name: 'Mike' },
];

const newData = [
  { id: 2, name: 'Jane' },
  { id: 4, name: 'Rick' },
  { id: 5, name: 'Jim' },
];

const usersMap = new Map();

oldData.forEach(user => usersMap.set(user.id, user));

newData.forEach(user => {
  const exists = usersMap.has(user.id);
  
  if (!exists) {
    usersMap.set(user.id, user);
  }
})

usersMap.forEach(user => console.log(user))

user_23444190
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