Short ES6 solution

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

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()); 

ES6 simplifies this:

let merge = (obj1, obj2) => ({...obj1, ...obj2}); 

Note that repeated keys will be merged, and the value of the second object will prevail and the repeated value of the first object will be ignored.

Example:

let obj1 = {id: 1, uniqueObj1Key: "uniqueKeyValueObj1", repeatedKey: "obj1Val"}; let obj2 = {id: 1, uniqueObj2Key: "uniqueKeyValueObj2", repeatedKey: "obj2Val"}; merge(obj1, obj2) // {id: 1, uniqueObj1Key: "uniqueKeyValueObj1", repeatedKey: "obj2Val", uniqueObj2Key: "uniqueKeyValueObj2"} merge(obj2, obj1) // {id: 1, uniqueObj2Key: "uniqueKeyValueObj2", repeatedKey: "obj1Val", uniqueObj1Key: "uniqueKeyValueObj1"} 

Complete solution (with Lodash, not Underscore)

var a1 = [{ id : 1, name : "test"}, { id : 2, name : "test2"}] var a2 = [{ id : 1, count : "1"}, {id : 2, count : "2"}] var merge = (obj1, obj2) => ({...obj1, ...obj2}); _.zipWith(a1, a2, merge) (2) [{…}, {…}] 0: {id: 1, name: "test", count: "1"} 1: {id: 2, name: "test2", count: "2"} 

If you have an array of arrays to merge you can do it like this:

var arrayOfArraysToMerge = [a1, a2, a3, a4]; //a3 and a4 are arrays like a1 and a2 but with different properties and same IDs. _.zipWith(...arrayOfArraysToMerge, merge) (2) [{…}, {…}] 0: {id: 1, name: "test", count: "1", extra1: "val1", extra2: 1} 1: {id: 2, name: "test2", count: "2", extra1: "val2", extra2: 2} 

reduce version.

var a3 = a1.concat(a2).reduce((acc, x) => { acc[x.id] = Object.assign(acc[x.id] || {}, x); return acc; }, {}); _.values(a3); 

I think it's common practice in functional language.

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" } ] 

Already there are many great answers, I'll just add another one which is from a real problem I needed to solve yesterday.

I had an array of messages with user ids, and one array of users containing users' names and other details. This is how I managed to add user details to the messages.

var messages = [{userId: 2, content: "Salam"}, {userId: 5, content: "Hello"},{userId: 4, content: "Moi"}]; var users = [{id: 2, name: "Grace"}, {id: 4, name: "Janetta"},{id: 5, name: "Sara"}]; var messagesWithUserNames = messages.map((msg)=> { var haveEqualId = (user) => user.id === msg.userId var userWithEqualId= users.find(haveEqualId) return Object.assign({}, msg, userWithEqualId) }) console.log(messagesWithUserNames) 

Vanilla JS solution

const a1 = [{ id : 1, name : "test"}, { id : 2, name : "test2"}] const a2 = [{ id : 1, count : "1"}, {id : 2, count : "2"}] const merge = (arr1, arr2) => { const temp = [] arr1.forEach(x => { arr2.forEach(y => { if (x.id === y.id) { temp.push({ ...x, ...y }) } }) }) return temp } console.log(merge(a1, a2)) 

Wanted to add this answer which is derived from @daisihi answer above. Main difference is that this uses the spread operator. Also, at the end I remove the id because it was not desirable in the first place.

const a3 = [...a1, ...a2].reduce((acc, x) => { acc[x.id] = {...acc[x.id] || {}, ...x}; return acc; }, {}); 

This part was taken from another post. removing a property from a list of objects in an array

const newArray = Object.values(a3).map(({id, ...keepAttrs}) => keepAttrs); 

Found other solutions failing for some cases, so writing a better one here

const a1 = [{ id : 1, name : "test"}, { id : 2, name : "test2"}] const a2 = [{ id : 3, count : "3"}, { id : 1, count : "1"}, {id : 2, count : "2"}] const mergeHelper = new Map(a1.map(x => [x.id, x])); for (const x of a2) { if (mergeHelper.has(x.id)) { const item = mergeHelper.get(x.id); mergeHelper.set(x.id, {...item, ...x}); } else { mergeHelper.set(x.id, x); } } const mergedList = [...mergeHelper.values()]; // For sorted array // const mergedSortedList = [...mergeHelper.values()].sort((a, b) => a.id - b.id); console.log(mergedList)

Using js Map is way faster than other approaches, helps when array length is huge.

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" } ]); }); 

try this

var a1 = [{ id : 1, name : "test"}, { id : 2, name : "test2"}] var a2 = [{ id : 1, count : "1"}, {id : 2, count : "2"}] let arr3 = a1.map((item, i) => Object.assign({}, item, a2[i])); console.log(arr3); 

How about this?

const mergeArrayObjects = (arr1: any[], arr2: any[], mergeByKey: string): any[] => { const updatedArr = []; for (const obj of arr1) { const arr1ValueInArr2 = arr2.find( a => a[mergeByKey] === obj[mergeByKey], ); if (arr1ValueInArr2) { updatedArr.push(Object.assign(obj, arr1ValueInArr2)); } else { updatedArr.push(obj); } } const mergeByKeyValuesInArr1 = arr1.map(a => a[mergeByKey]); const remainingObjInArr2 = arr2.filter(a => !mergeByKeyValuesInArr1.includes(a[mergeByKey]) ) return updatedArr.concat(remainingObjInArr2) } 

You can write a simple object merging function like this

function mergeObject(cake, icing) { var icedCake = {}, ingredient; for (ingredient in cake) icedCake[ingredient] = cake[ingredient]; for (ingredient in icing) icedCake[ingredient] = icing[ingredient]; return icedCake; } 

Next, you need to do use a double-loop to apply it to your data structre

var i, j, a3 = a1.slice(); for (i = 0; i < a2.length; ++i) // for each item in a2 for (j = 0; i < a3.length; ++i) // look at items in other array if (a2[i]['id'] === a3[j]['id']) // if matching id a3[j] = mergeObject(a3[j], a2[i]); // merge 

You can also use mergeObject as a simple clone, too, by passing one parameter as an empty object.

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

You can use Object.groupBy to do the grouping, and then use Array#reduce to merge its content.

It worth noting that Object.groupBy hasn't been widely suported yet. So it might not be a good idea to use in production environment by the time this answer has been posted.

var a1 = [{ id : 1, name : "test"}, { id : 2, name : "test2"}]; var a2 = [{ id : 1, count : "1"}, {id : 2, count : "2"}]; var merged = Object.values(Object.groupBy([...a1, ...a2], ({ id }) => id)).map(e => e.reduce((acc, cur) => ({ ...acc, ...cur })) ); console.log(merged);

If you have exactly the same number of items in both array with same ids you could do something like this.

const mergedArr = arr1.map((item, i) => { if (item.ID === arr2[i].ID) { return Object.assign({}, item, arr2[i]); } });