7.1 Examples: updating an object destructively and non-destructively

The following code shows a function that updates object properties destructively and uses it on an object.

function setPropertyDestructively(obj, key, value) {
  obj[key] = value;
  return obj;
}
const obj = {city: 'Berlin', country: 'Germany'};
setPropertyDestructively(obj, 'city', 'Munich');
assert.deepEqual(obj, {city: 'Munich', country: 'Germany'});

The following code demonstrates non-destructive updating of an object:

function setPropertyNonDestructively(obj, key, value) {
  const updatedObj = {};
  for (const [k, v] of Object.entries(obj)) {
    updatedObj[k] = (k === key ? value : v);
  }
  return updatedObj;
}
const obj = {city: 'Berlin', country: 'Germany'};
const updatedObj = setPropertyNonDestructively(obj, 'city', 'Munich');
// We have created an updated object:
assert.deepEqual(updatedObj, {city: 'Munich', country: 'Germany'});
// But we didn’t change the original:
assert.deepEqual(obj, {city: 'Berlin', country: 'Germany'});

Spreading makes setPropertyNonDestructively() more concise:

function setPropertyNonDestructively(obj, key, value) {
  return {...obj, [key]: value};
}

Both versions of setPropertyNonDestructively() update shallowly: They only change the top level of an object.