JavaScript merging objects by id

Question

What\'s the correct way to merge two arrays in Javascript?

I\'ve got two arrays (for example):

var a1 = [{ id : 1, name : \"test\"}, { id : 2, name :         


        

Answer 1

None of them worked for me. I wrote own:

const formatteddata=data.reduce((a1,a2)=>{

for (let t=0; t<a1.length; t++)
    {var id1=a1[t].id
            for (let tt=0; tt<a2.length; tt++)
                {var id2=a2[tt].id
                    if(id1==date2)
                      {a1[t]={...a1[t],...a2[tt]}}
                }
    }
return a1

})

works with any amount of arrays of objects in arrays, with varying length and not always coinsciding dates

Answer 2

Short ES6 solution

const a3 = a1.map(t1 => ({...t1, ...a2.find(t2 => t2.id === t1.id)}))

Answer 3

The lodash implementaiton:

var merged = _.map(a1, function(item) {
    return _.assign(item, _.find(a2, ['id', item.id]));
});

The result:

[  
   {  
      "id":1,
      "name":"test",
      "count":"1"
   },
   {  
      "id":2,
      "name":"test2",
      "count":"2"
   }
]

Answer 4

const a3 = a1.map(it1 => {
   it1.test = a2.find(it2 => it2.id === it1.id).test
   return it1
 })

Answer 5

Assuming IDs are strings and the order does not matter, you can

1. Create a hash table.
2. Iterate both arrays and store the data in the hash table, indexed by the ID. If there already is some data with that ID, update it with Object.assign (ES6, can be polyfilled).
3. Get an array with the values of the hash map.

var hash = Object.create(null);
a1.concat(a2).forEach(function(obj) {
    hash[obj.id] = Object.assign(hash[obj.id] || {}, obj);
});
var a3 = Object.keys(hash).map(function(key) {
    return hash[key];
});

In ECMAScript6, if the IDs are not necessarily strings, you can use Map:

var hash = new Map();
a1.concat(a2).forEach(function(obj) {
    hash.set(obj.id, Object.assign(hash.get(obj.id) || {}, obj))
});
var a3 = Array.from(hash.values());

Answer 6

A working TypeScript version:

export default class Merge {
  static byKey(a1: any[], a2: any[], key: string) {
    const res = a1.concat(a2).reduce((acc, x) => {
      acc[x[key]] = Object.assign(acc[x[key]] || {}, x);
      return acc;
    }, {});

    return Object.entries(res).map(pair => {
      const [, value] = pair;
      return value;
    });
  }
}

test("Merge", async () => {
  const a1 = [{ id: "1", value: "1" }, { id: "2", value: "2" }];
  const a2 = [{ id: "2", value: "3" }];

  expect(Merge.byKey(a1, a2, "id")).toStrictEqual([
    {
      id: "1",
      value: "1"
    },
    { id: "2", value: "3" }
  ]);
});