https://github.com/mongodb/js-bson/issues/356

Map Object accepts a non-string value for the key, but it looks like the serializer doesn't allow it.

The code below causes an error:

const mapObject = new Map()

mapObject.set({foo: 'bar'}, 'foo')
mapObject.set(undefined, 'foo')
mapObject.set(NaN, 'bar')
mapObject.set(3, 'baz')
//-> Map (4) {{…} => "foo", undefined => "foo", NaN => "bar", 3 => "baz"}

// use MongoDB Node.JS Driver
await collection.insertOne({
  data: mapObject,
})
//-> TypeError: argument must be a string

Functions such as serializeString take a key as an argument, but it is uncertain whether the Map Object's key has a string, thus buffer.write() may raise a TypeError.

It can work around that by converting the key to a string and passing it in the serializeInto, but I don't know if that's a good way, as it easily causes duplicate keys.