const obj = {c: 3}
const array = [{a: 2}, {b: 3, d:8}]
array.forEach(i => Object.assign(obj, i))
obj["a"] = 999
obj["b"] = 999
obj["d"] = 999
console.log(array) // [{a: 2}, {b: 3, d:8}]
array[0]["a"] = 2000
array[1]["b"] = 2000
array[2]["d"] = 2000
console.log(obj) // {c: 3, a: 999, b: 999, d: 999}