NodeJS: TypeError: Buffer is not a function (but it's a function!)

A bit baffled by this one. I'm using a pretty typical Node+Express setup, and many of my scripts are manipulating Buffer objects, which work fine basically everywhere. Except in one script, which defines a route for retrieving a document from an ElasticSearch data store. I'm trying to create a new Buffer object by decoding a base64-encoded string of the file data, and then send that to the browser. But even though a console.log immediately before the call to new Buffer(fildata,'base64') prints out a description of the Buffer function just as you'd expect it to, I get an exception "TypeError: Buffer is not a function".

The relevant code:

var Buffer = require('buffer');
// ... then in the route, inside the callback from the ElasticSearch get() method:
res.setHeader('Content-Type', doc["mimetype"]);
res.setHeader('Content-disposition', 'attachment; filename=' + doc["filename"]);
res.type(doc["mimetype"]);
console.log(Buffer); // Yep, definitely a function here!
res.send(new Buffer(doc["filedata"], 'base64'));

And the output from node (note the console.log output just before the exception is thrown):

{ Buffer:
   { [Function: Buffer]
     poolSize: 8192,
     isBuffer: [Function: isBuffer],
     compare: [Function: compare],
     isEncoding: [Function],
     concat: [Function],
     byteLength: [Function: byteLength] },
  SlowBuffer: [Function: SlowBuffer],
  INSPECT_MAX_BYTES: 50,
  kMaxLength: 2147483647 }
/Users/myusername/mynodeproject/routes/retrieve.js:50
                    res.send(new Buffer(doc["filedata"], 'base64'));
                             ^

TypeError: Buffer is not a function
    at /Users/myusername/mynodeproject/routes/retrieve.js:50:15
    at respond (/Users/myusername/mynodeproject/node_modules/elasticsearch/src/lib/transport.js:301:9)
    at checkRespForFailure (/Users/myusername/mynodeproject/node_modules/elasticsearch/src/lib/transport.js:239:7)
    at HttpConnector.<anonymous> (/Users/myusername/mynodeproject/node_modules/elasticsearch/src/lib/connectors/http.js:155:7)
    at IncomingMessage.wrapper (/Users/myusername/mynodeproject/node_modules/lodash/index.js:3095:19)
    at emitNone (events.js:72:20)
    at IncomingMessage.emit (events.js:166:7)
    at endReadableNT (_stream_readable.js:905:12)
    at doNTCallback2 (node.js:452:9)
    at process._tickCallback (node.js:366:17)

Solution

Reading your console output, the return value of require("buffer") is not a function. Instead, it's an object that has two methods: Buffer and SlowBuffer.

In your case, Buffer is that object with those two methods. You should instead do var Buffer = require('buffer').Buffer; (assuming you don't also need SlowBuffer).

As noted in another answer, you don't even need to require("buffer"). It is automatically available as a global in Node.