I just discovered that Node (tested: v0.8.23, current git: v0.11.3-pre) ignores any decoding errors in its Buffer handling, silently replacing any non-utf8 characters with '\ufffd' (the Unicode REPLACEMENT CHARACTER) instead of throwing an exception about the non-utf8 input. As a consequence, fs.readFile, process.stdin.setEncoding and friends mask a large class of bad input errors for you.
Example which doesn't fail but really ought to:
> notValidUTF8 = new Buffer([ 128 ], 'binary')
<Buffer 80>
> decodedAsUTF8 = notValidUTF8.toString('utf8') // no exception thrown here!
'�'
> decodedAsUTF8 === '\ufffd'
true
'\ufffd' is a perfectly valid character that can occur in legal utf8 (as the sequence ef bf bd), so it is non-trivial to monkey-patch in error handling based on this showing up in the result.
Digging a little deeper, it looks like this stems from node just deferring to v8's strings and that those in turn have the above behaviour, v8 not having any external world full of foreign-encoded data.
Are there node modules or otherwise that let me catch utf-8 decode errors, preferrably with context about where the error was discovered in the input string or buffer?
From node 8.3 on, you can use util.TextDecoder to solve this cleanly:
const util = require('util')
const td = new util.TextDecoder('utf8', {fatal:true})
td.decode(Buffer.from('foo')) // works!
td.decode(Buffer.from([ 128 ], 'binary')) // throws TypeError
This will also work in some browsers by using TextDecoder in the global namespace.