feat!(NODE-4461): remove Decimal128 toObject transformer (#526)

docs/upgrade-to-v5.md

@@ -79,8 +79,11 @@ We have set our typescript compilation target to `es2020` which aligns with our
 > **TL;DR**: TODO
 
 TODO(NODE-4771): serializeFunctions bug fix makes function names outside the ascii range get serialized correctly
-> This will preserve newer ECMAScript 2020 features like optional chaining, nullish coalescing, export * as ns, and dynamic import(...) syntax. It also means bigint literals now have a stable target below esnext.
 
 ### Remove `Map` export
 
 This library no longer polyfills [ES Map](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Map) and the export "Map" was removed. Users should migrate to using the global Map constructor available in all supported JS environments.
+
+### `Decimal128` `toObject()` mapper support removed
+
+`Decimal128` can no longer have a `toObject()` method added on to its prototype for mapping to a custom value. This feature was undocumented and inconsistent with the rest of our BSON types. At this time there is no direct migration: cursors in the driver support transformations via `.map`, otherwise the `Decimal128` instances will require manual transformation. There is a plan to provide a better mechanism for consistently transforming BSON values tracked in [NODE-4680](https://jira.mongodb.org/browse/NODE-4680), please feel free to add a vote or comment with a use case to help us land the feature in the most useful form.

src/parser/deserializer.ts

@@ -373,13 +373,7 @@ function deserializeObject(
  // Update index
  index = index + 16;
  // Assign the new Decimal128 value
- const decimal128 = new Decimal128(bytes) as Decimal128 | { toObject(): unknown };
- // If we have an alternative mapper use that
- if ('toObject' in decimal128 && typeof decimal128.toObject === 'function') {
- value = decimal128.toObject();
- } else {
- value = decimal128;
- }
+ value = new Decimal128(bytes);
  } else if (elementType === constants.BSON_DATA_BINARY) {
  let binarySize =
  buffer[index++] |

test/node/decimal128_tests.js

@@ -1174,37 +1174,6 @@ describe('Decimal128', function () {
  done();
  });
 
- it('Support toBSON and toObject methods for custom mapping', function (done) {
- // Create a custom object
- var MyCustomDecimal = function (value) {
- this.value = value instanceof Decimal128 ? value.toString() : value;
- };
-
- MyCustomDecimal.prototype.toBSON = function () {
- return Decimal128.fromString(this.value);
- };
-
- // Add a custom mapper for the type
- const saveToObject = Decimal128.prototype.toObject;
- try {
- Decimal128.prototype.toObject = function () {
- return new MyCustomDecimal(this);
- };
-
- // Test all methods around a simple serialization at object top level
- var doc = { value: new MyCustomDecimal('1') };
- var buffer = BSON.serialize(doc);
- var back = BSON.deserialize(buffer);
- expect(back.value instanceof MyCustomDecimal).to.be.ok;
- expect('1').to.equal(back.value.value);
- } finally {
- // prevent this test from breaking later tests which may re-use the same class
- Decimal128.prototype.toObject = saveToObject;
- }
-
- done();
- });
-
  it('accepts strings in the constructor', () => {
  expect(new Decimal128('0').toString()).to.equal('0');
  expect(new Decimal128('00').toString()).to.equal('0');