fix(serializeWithBufferAndIndex): write documents to start of intermediate buffer

Author: kmahar

Diff for: lib/bson/bson.js

@@ -96,15 +96,15 @@ BSON.prototype.serializeWithBufferAndIndex = function(object, finalBuffer, optio
     buffer,
     object,
     checkKeys,
-    startIndex || 0,
+    0,
     0,
     serializeFunctions,
     ignoreUndefined
   );
   buffer.copy(finalBuffer, startIndex, 0, serializationIndex);
 
   // Return the index
-  return serializationIndex - 1;
+  return startIndex + serializationIndex - 1;
 };
 
 /**

Diff for: test/node/serialize_with_buffer_tests.js

@@ -1,6 +1,6 @@
 'use strict';
 
-var createBSON = require('../utils'),
+let createBSON = require('../utils'),
   expect = require('chai').expect;
 
 describe('serializeWithBuffer', function() {
@@ -28,4 +28,40 @@ describe('serializeWithBuffer', function() {
     expect({ a: 1 }).to.deep.equal(doc);
     done();
   });
+
+  it('correctly serialize 3 different docs into buffer using serializeWithBufferAndIndex', function(
+    done
+  ) {
+    const MAXSIZE = 1024 * 1024 * 17;
+    var bson = createBSON();
+    let bf = new Buffer(MAXSIZE);
+
+    const data = [
+      {
+        a: 1,
+        b: new Date('2019-01-01')
+      },
+      {
+        a: 2,
+        b: new Date('2019-01-02')
+      },
+      {
+        a: 3,
+        b: new Date('2019-01-03')
+      }
+    ];
+
+    let idx = 0;
+    data.forEach(item => {
+      idx =
+        bson.serializeWithBufferAndIndex(item, bf, {
+          index: idx
+        }) + 1;
+    });
+
+    expect(bson.deserialize(bf.slice(0, 23))).to.deep.equal(data[0]);
+    expect(bson.deserialize(bf.slice(23, 46))).to.deep.equal(data[1]);
+    expect(bson.deserialize(bf.slice(46, 69))).to.deep.equal(data[2]);
+    done();
+  });
 });