Merge 2 object with same key, value from 2 array

Question

I want to merge 2 object with same key, value from 2 array, something like this:

var arr1 = [
    { a: "a", 1: 1, 2: 2 },
    { a: "b", 1: 1, 2: 3 }
];

var arr2 = [
    { a: "a", 3: 123 },
    { a: "b", 3: 4411 }
];

var arr3 = _.map(arr1, function(a1) {
    var a3 = {};

    _.map(arr2, function(a2) {
        if (a1.a == a2.a) {
            a3 = _.extend(a1, a2);
        }
    })

    return a3
});

result:

arr3 = [ 
  { '1': 1, '2': 2, '3': 123, a: 'a' },
  { '1': 1, '2': 3, '3': 4411, a: 'b' } 
]

Does it look stupid? Are there any others ways to do this? Thanks for reading.

Answer 1

Use a lodash chain to concat the arrays, group similar objects, and then merge each group to a single object:

var arr1 = [{ a: "a", 1: 1, 2: 2 }, { a: "b", 1: 1, 2: 3 }];
var arr2 = [{ a: "a", 3: 123 }, { a: "b", 3: 4411 }];

var result = _(arr1)
  .concat(arr2) // concat the 2nd array
  .groupBy('a') // group by the identical key
  .map(_.spread(_.curry(_.merge, {}))) // left currey merge to to create a new empty object, and spread the group as parameters
  .value();
  
console.log(result);

<script src="https://cdnjs.cloudflare.com/ajax/libs/lodash.js/4.17.4/lodash.min.js"></script>

With ES6 you can use Array#reduce to collect the similar objects in a Map, then get the Map#values iterator, and use the spread syntax to convert to an array:

const arr1 = [{ a: "a", 1: 1, 2: 2 }, { a: "b", 1: 1, 2: 3 }];
const arr2 = [{ a: "a", 3: 123 }, { a: "b", 3: 4411 }];

const result = [...arr1.concat(arr2) // concat the arrays
  .reduce((m, o) => m.set(o.a, Object.assign(m.get(o.a) || {}, o)), // use a map to collect similar objects
  new Map()
).values()]; // get the values iterator of the map, and spread into a new array
  
console.log(result);

Answer 2

you can do

var arr1 = [
    { a: "a", 1: 1, 2: 2 },
    { a: "b", 1: 1, 2: 3 }
];

var arr2 = [
    { a: "a", 3: 123 },
    { a: "b", 3: 4411 }
];

let result = arr1.map((e) => {
    for(let element of arr2){
        if(e.a == element.a) Object.assign(e, element);
    }
    return e;
});
console.log(result);