How can I merge two arrays of objects in JavaScript - Stack Overflow

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I have a simple question. I have two arrays A and B, I want to retain A objects if B has the same ID. For example:

const A = [{id: "price", value: "1"}]

const B = [{id: "price", value: "0"}, {id: "number", value: "0"}]

Expected result:

[{id: "price", value: "1"}, {id: "number", value: "0"}}]

How can I do this?

I tried to map A and foreach B inside A but it didn't work.

I have a simple question. I have two arrays A and B, I want to retain A objects if B has the same ID. For example:

const A = [{id: "price", value: "1"}]

const B = [{id: "price", value: "0"}, {id: "number", value: "0"}]

Expected result:

[{id: "price", value: "1"}, {id: "number", value: "0"}}]

How can I do this?

I tried to map A and foreach B inside A but it didn't work.

• javascript
• arrays
• object
• ecmascript-6

Share Improve this question Follow edited May 9, 2019 at 3:13 chatnoir 2,30311 gold badge1616 silver badges1717 bronze badges asked May 9, 2019 at 1:19 Tiago CastroTiago Castro 42166 silver badges2020 bronze badges 8

• Do you want to replace A's objects or B's objects? The expected result is confusing – ibrahim mahrir Commented May 9, 2019 at 1:24
• The term is "merging" and has been covered plenty of times... – Heretic Monkey Commented May 9, 2019 at 1:24
• I have edit. Thats right, I want to merge these two arrays into one – Tiago Castro Commented May 9, 2019 at 1:25
• If A and B have same ID, do you want to keep A or B? Your question does not match your expected result. – yqlim Commented May 9, 2019 at 1:26
• 2 Possible duplicate of es6 merge two array of objects and override the existing object – Heretic Monkey Commented May 9, 2019 at 1:27

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3 Answers 3

Sorted by: Reset to default 8

const result = A.concat(B.filter(bo => A.every(ao => ao.id != bo.id)));

Concatenate all the objects from A with objects from B that aren't in A (which is done by filtering only objects from B where there isn't an object in A with the same id).

Example:

const A = [{id: "price", value: "1"}];

const B = [{id: "price", value: "0"}, {id: "number", value: "0"}];

const result = A.concat(B.filter(bo => A.every(ao => ao.id != bo.id)));

console.log(result);

You'd use reduce on the merged array - also turn the value into a number:

const A = [{id: "price", value: "1"}];
const B = [{id: "price", value: "0"}, {id: "number", value: "0"}];
const res = Object.values([...A, ...B].reduce((acc, { id, value }) => {
  if (acc[id]) acc[id].value += parseInt(value);
  else acc[id] = { id, value: parseInt(value) };
  return acc;
}, {}));
console.log(res);

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Another option that you could try (I believe it would be O(n) ) is to convert arrays to objects with id as key then extend (jquery.extend or pure js implementation) then convert the merged object back to array.

const A = [{id: "price", value: "1"}];

const B = [{id: "price", value: "0"}, {id: "number", value: "0"}];

//convert arrays to objects
var Bx = {};
B.forEach(i => Bx[i.id] = i);

var Ax = {};
A.forEach(i => Ax[i.id] = i);

//copy all matching id properties from A to B
A.forEach(i => Bx[i.id] = Ax[i.id]);

//convert the merged object to array
var C = [];
Object.getOwnPropertyNames(Bx).forEach(i => C.push(Bx[i]));

console.log(C);

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