diff --git a/test/node/bson_test.js b/test/node/bson_test.js
index d7edc17f..99693649 100644
--- a/test/node/bson_test.js
+++ b/test/node/bson_test.js
@@ -298,80 +298,6 @@ describe('BSON', function () {
     done();
   });
 
-  /**
-   * @ignore
-   */
-  it('Should correctly ignore undefined values in arrays', function (done) {
-    var doc = { doc: { notdefined: undefined } };
-    var serialized_data = BSON.serialize(doc, {
-      ignoreUndefined: true
-    });
-    var serialized_data2 = Buffer.alloc(
-      BSON.calculateObjectSize(doc, {
-        ignoreUndefined: true
-      })
-    );
-    BSON.serializeWithBufferAndIndex(doc, serialized_data2, {
-      ignoreUndefined: true
-    });
-
-    assertBuffersEqual(done, serialized_data, serialized_data2, 0);
-    var doc1 = BSON.deserialize(serialized_data);
-
-    expect(undefined).to.deep.equal(doc1.doc.notdefined);
-    done();
-  });
-
-  it('Should correctly serialize undefined array entries as null values', function (done) {
-    var doc = { doc: { notdefined: undefined }, a: [1, 2, undefined, 3] };
-    var serialized_data = BSON.serialize(doc, {
-      ignoreUndefined: true
-    });
-    var serialized_data2 = Buffer.alloc(
-      BSON.calculateObjectSize(doc, {
-        ignoreUndefined: true
-      })
-    );
-    BSON.serializeWithBufferAndIndex(doc, serialized_data2, {
-      ignoreUndefined: true
-    });
-    assertBuffersEqual(done, serialized_data, serialized_data2, 0);
-    var doc1 = BSON.deserialize(serialized_data);
-    expect(undefined).to.deep.equal(doc1.doc.notdefined);
-    expect(null).to.equal(doc1.a[2]);
-    done();
-  });
-
-  it('Should correctly serialize undefined array entries as undefined values', function (done) {
-    var doc = { doc: { notdefined: undefined }, a: [1, 2, undefined, 3] };
-    var serialized_data = BSON.serialize(doc, {
-      ignoreUndefined: false
-    });
-    var serialized_data2 = Buffer.alloc(
-      BSON.calculateObjectSize(doc, {
-        ignoreUndefined: false
-      })
-    );
-    BSON.serializeWithBufferAndIndex(doc, serialized_data2, {
-      ignoreUndefined: false
-    });
-
-    // console.log("======================================== 0")
-    // console.log(serialized_data.toString('hex'))
-    // console.log(serialized_data2.toString('hex'))
-
-    assertBuffersEqual(done, serialized_data, serialized_data2, 0);
-    var doc1 = BSON.deserialize(serialized_data);
-    var doc2 = BSON.deserialize(serialized_data2);
-    // console.log("======================================== 0")
-    // console.dir(doc1)
-    // console.dir(doc2)
-
-    expect(null).to.deep.equal(doc1.doc.notdefined);
-    expect(null).to.deep.equal(doc2.doc.notdefined);
-    done();
-  });
-
   /**
    * @ignore
    */
diff --git a/test/node/bson_undefined.test.ts b/test/node/bson_undefined.test.ts
new file mode 100644
index 00000000..b902450c
--- /dev/null
+++ b/test/node/bson_undefined.test.ts
@@ -0,0 +1,108 @@
+import { expect } from 'chai';
+import { bufferFromHexArray } from './tools/utils';
+import { BSON } from '../register-bson';
+import { BSON_DATA_NULL } from '../../src/constants';
+
+describe('BSON undefined', () => {
+  const KEY_A = '6100';
+  const KEY_0 = '3000';
+  const KEY_1 = '3100';
+  const KEY_2 = '3200';
+
+  describe('when deserialize is given BSON bytes with undefined value', function () {
+    it('returns a javascript undefined value', () => {
+      const bsonDocWithUndefined = bufferFromHexArray([
+        '06', // BSON undefined
+        KEY_A
+      ]);
+      const doc = BSON.deserialize(bsonDocWithUndefined);
+      expect(doc).to.have.own.property('a').that.is.undefined;
+    });
+  });
+
+  describe('when serialize is given a javascript object that contains undefined', () => {
+    describe('when ignoreUndefined is set to false', function () {
+      it('serializes to document with a set to BSON null (type=10)', () => {
+        const jsObject = { a: undefined };
+        const bytes = BSON.serialize(jsObject, { ignoreUndefined: false });
+        expect(bytes).to.have.lengthOf(8);
+        const elements = BSON.onDemand.parseToElements(bytes);
+        expect(elements).to.have.lengthOf(1);
+        expect(elements[0][0]).to.deep.equal(BSON_DATA_NULL);
+      });
+    });
+
+    describe('when ignoreUndefined is set to true', function () {
+      it('serializes to empty document', () => {
+        const jsObject = { a: undefined };
+        const bytes = BSON.serialize(jsObject, { ignoreUndefined: true });
+        expect(bytes).to.deep.equal(Uint8Array.of(5, 0, 0, 0, 0));
+      });
+    });
+
+    describe('when ignoreUndefined is unset', function () {
+      it('serializes to empty document', () => {
+        const jsObject = { a: undefined };
+        const bytes = BSON.serialize(jsObject);
+        expect(bytes).to.deep.equal(Uint8Array.of(5, 0, 0, 0, 0));
+      });
+    });
+  });
+
+  describe('when undefined appears inside an array', function () {
+    describe('when ignoreUndefined is set to true', function () {
+      it('serializes undefined values as null', function () {
+        // because this would change the size of the array
+        const doc = { a: [1, undefined, 3] };
+        const bytes = BSON.serialize(doc, { ignoreUndefined: true });
+        expect(bytes).to.deep.equal(
+          bufferFromHexArray([
+            '04', // array
+            KEY_A,
+            bufferFromHexArray([
+              ...['10', KEY_0, '01000000'], // int "0" = 1
+              ...['0A', KEY_1], // null "1"
+              ...['10', KEY_2, '03000000'] // int "2" = 3
+            ]).toString('hex')
+          ])
+        );
+      });
+    });
+
+    describe('when ignoreUndefined is set to false', function () {
+      it('serializes undefined values as null', function () {
+        const doc = { a: [1, undefined, 3] };
+        const bytes = BSON.serialize(doc, { ignoreUndefined: false });
+        expect(bytes).to.deep.equal(
+          bufferFromHexArray([
+            '04', // array
+            KEY_A,
+            bufferFromHexArray([
+              ...['10', KEY_0, '01000000'], // int "0" = 1
+              ...['0A', KEY_1], // null "1"
+              ...['10', KEY_2, '03000000'] // int "2" = 3
+            ]).toString('hex')
+          ])
+        );
+      });
+    });
+
+    describe('when ignoreUndefined is unset', function () {
+      it('serializes undefined values as null', function () {
+        const doc = { a: [1, undefined, 3] };
+        const bytes = BSON.serialize(doc);
+        expect(bytes).to.deep.equal(
+          bufferFromHexArray([
+            '04', // array
+            KEY_A,
+            bufferFromHexArray([
+              ...['10', KEY_0, '01000000'], // int "0" = 1
+              ...['0A', KEY_1], // null "1"
+              ...['10', KEY_2, '03000000'] // int "2" = 3
+            ]).toString('hex')
+          ])
+        );
+      });
+    });
+  });
+});
diff --git a/test/node/parser/serializer.test.ts b/test/node/parser/serializer.test.ts
index c06161cc..6af3f27a 100644
--- a/test/node/parser/serializer.test.ts
+++ b/test/node/parser/serializer.test.ts
@@ -26,6 +26,7 @@ describe('serialize()', () => {
   });
 
   it('does not turn nested nulls into empty documents', () => {
+    // In JS typeof null is 'object' so it is possible it could be misinterpreted as an object with no keys
     const nestedNull = bufferFromHexArray([
       '0A', // null type
       '6100', // 'a\x00'