feat(NODE-4789): support Map stringification in EJSON (#567)

src/extended_json.ts

@@ -17,7 +17,7 @@ import { Long } from './long';
 import { MaxKey } from './max_key';
 import { MinKey } from './min_key';
 import { ObjectId } from './objectid';
-import { isDate, isRegExp } from './parser/utils';
+import { isDate, isRegExp, isMap } from './parser/utils';
 import { BSONRegExp } from './regexp';
 import { BSONSymbol } from './symbol';
 import { Timestamp } from './timestamp';
@@ -190,6 +190,18 @@ function getISOString(date: Date) {
 
 // eslint-disable-next-line @typescript-eslint/no-explicit-any
 function serializeValue(value: any, options: EJSONSerializeOptions): any {
+  if (value instanceof Map || isMap(value)) {
+    const obj: Record<string, unknown> = Object.create(null);
+    for (const [k, v] of value) {
+      if (typeof k !== 'string') {
+        throw new BSONError('Can only serialize maps with string keys');
+      }
+      obj[k] = v;
+    }
+
+    return serializeValue(obj, options);
+  }
+
   if ((typeof value === 'object' || typeof value === 'function') && value !== null) {
     const index = options.seenObjects.findIndex(entry => entry.obj === value);
     if (index !== -1) {

test/node/extended_json.test.ts

@@ -3,6 +3,7 @@ const EJSON = BSON.EJSON;
 import * as vm from 'node:vm';
 import { expect } from 'chai';
 import { BSONVersionError, BSONRuntimeError } from '../../src';
+import { BSONError } from '../register-bson';
 
 // BSON types
 const Binary = BSON.Binary;
@@ -583,4 +584,42 @@ describe('Extended JSON', function () {
       })
     ).to.throw(BSONVersionError, /Unsupported BSON version/i);
   });
+
+  context('Map objects', function () {
+    it('serializes an empty Map object', () => {
+      const input = new Map();
+      expect(EJSON.stringify(input)).to.equal('{}');
+    });
+
+    it('serializes a nested Map object', () => {
+      const input = new Map([
+        [
+          'a',
+          new Map([
+            ['a', 100],
+            ['b', 200]
+          ])
+        ]
+      ]);
+
+      const str = EJSON.stringify(input);
+      expect(str).to.equal('{"a":{"a":100,"b":200}}');
+    });
+
+    it('serializes a Map with one string key', () => {
+      const input = new Map([['a', 100]]);
+
+      const str = EJSON.stringify(input);
+      expect(str).to.equal('{"a":100}');
+    });
+
+    it('throws BSONError when passed Map object with non-string keys', () => {
+      const input: Map<number, unknown> = new Map([
+        [1, 100],
+        [2, 200]
+      ]);
+
+      expect(() => EJSON.stringify(input)).to.throw(BSONError);
+    });
+  });
 });