concat-arrays-uniq-with-comparation.js

/*
  Just experiments and some thoughts.
  Better use for/foreach over arrays and check if target arr already contains objects, something like:
  
  const target = [...one]

  for (current of two) {
    const duplicated = target.find(item => item.id === current.id);

    !duplicated && target.push(current);

    // choose which element to put into the array
    if (duplicated && current.value > duplicated.value)
      target[target.indexOf(duplicated)] = current;
  }
  
*/

const one = [
  { id: 1, value: 11 },
  { id: 2, value: 23 },
  { id: 3, value: 42 },	//dup
  { id: 4, value: 21 }, //dup prior
]

const two = [
  { id: 3, value: 55 }, //dup prior
  { id: 4, value: 5 }, //dup
  { id: 5, value: 123 },
  { id: 6, value: 66 },
]

const arr = [...one, ...two]

const compare = (o1, o2) => { return o1.value >= o2.value ? o1 : o2 }

function filterWithComparison(arr, compareFunc, key = 'id') {
    return arr.reduce((acc, current) => {

      function compareAndReplace(duplicated) {
        const obj = compareFunc(current, duplicated);
        acc[acc.indexOf(duplicated)] = obj;
      }

      //check
      const duplicated = acc.find(item => item[key] === current[key]);

      duplicated && compareAndReplace(duplicated);

      return !duplicated ? [...acc, current ] : acc;
    }, []);
}

const filtered = filterWithComparison(arr, compare)

console.log(filtered)