Lessons Learned Coding Flatbuffers in Javascript

This page is the output of wrestling with Flatbuffers in Javascript. Long winded but an attempt to describe how Flatbuffers need to be used with Javascript. The information was correct when written (April 2017).


  • These lessons are for Flatbuffers in JavaScript. Your speed will vary with other target languages.
  • One must create one flatbuffer.Buffer() for each output record being created and that buffer must be used when building all the structs and tables that go into that output record;
  • On input, once the root structure has been read, all the included structures and tables can be referenced with the access members and there is a special (and not well documented) syntax for specifying tables from unions.

Buffers and Building

In building the browser based version of Basil, the team decided the communication protocol serializer would be Flatbuffers for its presumed speed and newness. That meant coding Flatbuffers in JavaScript.

Flatbuffers is one of those protocol systems (like Protobufs) where a schema is written and then compiled into binding code for a particular language. Happily, Flatbuffers has a compiler for JavaScript.

Thinking object-orientedly, I defined a communication record that contained many struct’s and table’s (the two forms of structured data in Flatbuffers schemas), compiled same, and started coding the JavaScript. I wanted to design the communication stack with a command layer that sits on top of a flow layer. Still thinking in an object-oriented fashion, I wrote code where I built the different tables (for commands and message flow) in different layers of the code and then combined them into the final message to send.

Nothing worked.

So, I spent much time staring at the limited JavaScript documentation (while Flatbuffers has a JavaScript compiler, that language is obviously not the primary target) and then staring at the generated code.

Inspecting the library and the generated code (I love open source), I realized something that is not very well explained in the documentation – the record that will eventually be sent is being built while the structs and tables are being constructed (with the ‘start’, ‘add’, and ‘end’ methods). The fact that all the tables have an object-oriented type of design mislead me into thinking that the building of each table is independent. So, my first solution created a new flatbuffers.Buffer() for each table and then expected to use the output of the ‘end’ operation to give an object that would be added to the enclosing table.

As a concrete example, take this schema:

namespace frog;

table coord {
    x: int;
    y: int;
table item {
    position: coord;
    name: string;
table animation {
    animationFile: string;
    applyTo: string;
table messageSequence {
    seq: int;
    timeSent: long;
union msgMsg {
table message {
    sequence: messageSequence;
    msg: msgMsg;

This is a simple definition of a message that includes some flow information (the message sequence number and time sent) and either an item or an animation specification.

My initial implementation was made up of a bunch of classes which were like:

var builder = new flatbuffers.Builder();
frog.coord.addX(builder, 23);
frog.coord.addY(builder, 44);
var builtCoord = frog.coord.endcoord(builder);

Figuring the ‘end’ function was completing the object and giving me a handle to same. Eventually, I would create the output message with:

var builder = new flatbuffers.Builder();
frog.message.addSeq(builder, builtSeq);
frog.message.addMsg(builder, builtItem);
var builtMsg = frog.message.endmessage(builder);

This didn’t work because what is really happening with the ‘add’ operations is the placement of offsets and data into the output message in the builder. That is, for performance reasons and to eliminate copies and transforms, the message is constructed as the tables are built. The thing I didn’t get from the examples on the web site was that there needs to be only one flatbuffers.Builder for each message sent and it must be used to build all the structs and tables that will be in that message. This fact is hidden in discussions about building tables non-recursively and building up vectors before building the base table.

So, the solution is to create one flatbuffers.Builder per message and pass that around to all the routines that create the message contents. For instance:

var builder = new flatbuffers.Builder();

// Create the first table (which gets put into the buffer in the Builder)
frog.coord.addX(builder, 23);
frog.coord.addY(builder, 44);
var builtCoord = frog.coord.endcoord(builder);

// Create another table using the same Builder)
frog.item.addposition(builder, builtCoord);
frog.item.addname(builder, builder.createString('froggishItem'));
var builtItem = frog.item.enditem(builder);

// Another table for the flow information
frog.messageSequence.addseq(builder, globalMessageSequenceNumber++);
frog.messageSequence.addtimeSent(builder, Date.now());
var builtSeq = frog.messageSequence.endmessageSequence(builder);

// Finally, create the enclosing table and add the offsets from the 'end' methods
frog.message.addSequence(builder, builtSeq);
frog.message.addMsg(builder, builtItem);
frog.message.addMsgType(builder, frog.msgMsg.item);
var builtMsg = frog.message.endmessage(builder);

// Finish the message buffer with all the tables in the Builder.

// Write out the completed message as a byte array

The way I now think about it is that the creation of the flatbuffers.Builder() is allocating the buffer that the output message will be constructed into. All the ‘start’, ‘end’, and ‘add’ operations are storing the information about that table into the Builder’s buffer. The final finish call does any fixup and the record is ready for sending.

The additional complication for my code was the communication layer organization. I ended up with a control layer that builds the command table (like the ‘item’ table in the above example) and it was the builder that was passed to the flow layer. The flow layer then added the flow control tables before passing the whole builder to the transport layer.

Getting Tables from Unions

Another head scratcher was, on reception, how to get the table out of the union. This is what I found:

Using the msg variable of type msgMsg in the above schema, a union definition creates an enumeration and two variables in the generated Javascript code:

  • addMsg for putting the msgMsg table into the message;
  • addMsgType for putting a code of what type of table was used for msgMsg;
  • msgMsg enumeration with codes for each of the table types that can be in a msg.

If the target language is C++, the extra variable for the type is named msg_type but, for Javascript, the name is msgType. The code must set the type when filling the union variable (see above).

On the receiving side, the union can be extracted thusly:

var buff = bufferOfRawMessageAsByteArray;
// ByteBuffer is the input equivilent to Builder
var fbBuff = new flatbuffers.ByteBuffer(buff);
// Get the buffer as a 'message'
var receivedFb = frog.message.getRootAsmessage(fbBuff);

// Note that the variable fetching methods are functions
var seqTable = receivedFb.sequence();
// If the table was not added by the sender, 'undefined' is returned
if (seqTable != undefined) {
    console.out('Sequence number = ' + seqTable.seq());

if (receivedFb.msgType() != undefined) {
    switch(receivedFb.msgType()) {
        case frog.msgMsg.NONE:
            // There was no table added by the sender
        case frog.msgMsg.item:
            var itemTable = receivedFb.msg(new frog.item());
            console.out('Received item named ' + itemTable.name());
        case frog.msgMsg.animation:
            var animationTable = receivedFb.msg(new frog.animation());
            console.out('Received animation from file ' + animationTable.animationFile());

The trick for Javascript is to add a ‘new’ of an empty table type in the fetcher for the union field. What the fetcher for a union does is initialize the empty table passed pointing into the union field. As shown, the extra type variable tells the receiver what type is in the union.

There you go, a very long winded explanation of Javascript and Flatbuffers.