Javascript: How to merge two objects and sum the values of the same key?

Question

const map1 = { "a": 10, "b": 6 };
const map2 = { "a": 10, "b": 6, "c": 7, "d": 8 };

I want to merge them and sum their values if they have the same key:

const result = { "a": 20, "b": 12, "c": 7, "d": 8 };

How can you achieve this without using any external library or ++? Thanks!

Answer 1

Using Array#reduce and Object#entries:

const 
  map1 = { "a": 10, "b": 6 },
  map2 = { "a": 10, "b": 6, "c": 7, "d": 8 };

// iterate over map2 entries with acc set to map1 at start
const merged = Object.entries(map2).reduce((acc, [key, value]) => 
  // if key is already in map1, add the values, otherwise, create new pair
  ({ ...acc, [key]: (acc[key] || 0) + value })
, { ...map1 });

console.log(merged);

Answer 2

I came up with this code, more readable and simpler though longer version const map1 = { "a": 10, "b": 6 , x: 22}; const map2 = { "a": 10, "b": 6, "c": 7, "d": 8 };

function addMerge(obj1, obj2){
  let res = {}
  let A = Object.keys(obj1)
  let B = Object.keys(obj2)
  if(A.length > B.length){
    Object.keys(obj1).forEach((k, i) => {
      if(obj2.hasOwnProperty(k))
        res[k] = obj2[k] + obj1[k]
      else
        if(B[i] !== undefined){
          res[k] = obj1[k]
          res[B[i]] = obj2[B[i]]
      }
        else
          res[k] =  obj1[k]

    });

  }
  else{
  Object.keys(obj2).forEach((k, i) => {

  if(obj1.hasOwnProperty(k))
    res[k] = obj2[k] + obj1[k]
  else
    if(A[i] !== undefined){
      res[k] = obj2[k]
      res[A[i]] = obj1[A[i]]
    }
    else
      res[k] = obj2[k]
});
}

console.log("Result", res)

}

addMerge(map1, map2)