object-merge.js

const defaultArr = [
  { day: 1, from: undefined, to: undefined },
  { day: 2, from: undefined, to: undefined },
  { day: 3, from: undefined, to: undefined },
  { day: 4, from: undefined, to: undefined },
  { day: 5, from: undefined, to: undefined },
  { day: 6, from: undefined, to: undefined },
  { day: 7, from: undefined, to: undefined }
];

const customArr = [
  { day: 1, from: "12:00", to: "15:00" },
  { day: 2, from: "13:00", to: "15:00" }
];

function objectMerge(defaultArr, customArr) {
  const finalArr = [];
  const customDayValues = [];

  for (item of customArr) {
    customDayValues.push(item.day);
  }

  for (item of defaultArr) {
    if (customDayValues.includes(item.day)) {
      customArr.map(customItem => {
        if (customItem.day === item.day) {
          finalArr.push(customItem);
        }
      });
    } else {
      finalArr.push(item);
    }
  }

  return finalArr;
}

const finalArr = objectMerge(defaultArr, customArr);
console.log(finalArr);