merge.js

// merge an array of objects
const data = [ {a: 1}, {b: 2}, {c: 3} ]
const merged = data.reduce((res, obj) => ({...res, ...obj}), {})
console.log(merged) // => { a: 1, b: 2, c: 3  }

// merge an array of objects by property
const toMerge = [
  { id: 1, value: 'a', },
  { id: 2, value: 'b', },
  { id: 3, value: 'c' },
  { id: 1, score: 1 },
  { id: 2, score: '2' },
]
const mergedByProperty = toMerge.reduce((result, obj) => ({
  ...result,
  [obj.id]: {
    ...result[obj.id],
    ...obj
  }
}), {})
console.log(mergedByProperty) // =>
/*
 *{ '1': { id: 1, value: 'a', score: 1 },
 *  '2': { id: 2, value: 'b', score: '2' },
 *  '3': { id: 3, value: 'c' } }
 */