Issue

• ES Map throws on undefined values
• ignoreUndefined has no effect, the error is thrown regardless
• Objects and arrays have if (value === undefined) handling case, Map is missing the same handling

Steps to reproduce?

> BSON.serialize(new Map().set('a', undefined))
Uncaught TypeError: Cannot read properties of undefined (reading '_bsontype')
    at serializeInto (/Users/neal/code/drivers/js/mongodb/node_modules/bson/lib/bson.cjs:3519:28)
    at Object.serialize (/Users/neal/code/drivers/js/mongodb/node_modules/bson/lib/bson.cjs:3997:32)

User Impact

• Impacts bson 1.1.6+
• Anyone using ES Maps for their documents
• Index API and client metadata notably use ES Map - they're not effected b/c they do not have undefined values

Dependencies

• This needs to be fixed in a patch to BSON and pulled into the driver

Unknowns

• Should we merge the Array, Object, and Map code paths?
  • Known risks? sparse arrays are special case
  • Unknown risks?

Acceptance Criteria

Implementation Requirements

• Add handling for undefined values from Map
  • Align with object, ignoreUndefined controls whether or not to include the value in the BSON bytes
  • if Undefined is to be included is MUST be serialized as a BSON null
    • BSON undefined is deprecated and no longer produced by our or any BSON library under normal circumstances

Testing Requirements

• Test for true, false, and default setting of ignoreUndefined
• Add tests for other containers (object, array) or reorganize existing ones

Documentation Requirements

• N/A

Follow Up Requirements

• Follow up with backport considerations