Refs: nodejs/node#8169, nodejs/node#7152.

Two options here:

• Use the new Buffer API — Buffer.alloc()/Buffer.from()/Buffer.allocUnsafe() (requires a shim for v0.10/v0.12 and older v4.x versions prior to v4.5.0).
• Use new Buffer() for the time being — just add the new keyword everywhere. You should manually check that everything is safe in this case, see the links below for more explanation. That might be hard-deprecated at some later point.

More background:

• https://github.com/ChALkeR/notes/blob/master/Lets-fix-Buffer-API.md
• https://github.com/ChALkeR/notes/blob/master/Buffer-knows-everything.md
• buffer: runtime-deprecate Buffer constructor nodejs/node#7152 (comment)

Quick grep (you should better re-check):

duplexify-3.4.5.tgz/test.js:279:      t.same(chunk, Buffer('hello world'))
duplexify-3.4.5.tgz/test.js:290:    dup.write(Buffer('hello world'))

The grep above only includes the lines that call Buffer() without the new keyword, if you choose to move to the new API — you should probably also replace new Buffer(…) calls.