Gulp, vinyl-fs and error "TypeError: isStream is not a function" - node.js

Question

I have a gulp file that up until relatively recently was working fine, but has started just throwing an error. I'm unsure of the best way to debug this, and google has not been of any help, so am looking for ideas on what might be wrong. The gulpfile has the following task defined (I've removed some other tasks to try and simplify this)
/// <binding AfterBuild='moveToCss' />
var gulp = require('gulp');
var paths = {
npmSrc: "./node_modules/",
cssTarget: "./wwwroot/css/",
};
var cssToMove = [
paths.npmSrc + '/bootstrap/dist/css/bootstrap.min.css',
paths.npmSrc + '/datatables.net-bs/css/dataTables.bootstrap.css',
paths.npmSrc + '/datatables.net-select-bs/css/select.bootstrap.css'
];
gulp.task('moveToCss', function () {
return gulp.src(cssToMove).pipe(gulp.dest(paths.cssTarget));
});
but when I run this I get the following error:
[13:52:27] Using gulpfile C:\MyProject\gulpfile.js
[13:52:27] Starting 'moveToCss'...
gulp : C:\MyProject\node_modules\gulp\node_modules\vinyl-fs\node_modules\vinyl\i
ndex.js:142
At line:1 char:1
+ gulp moveToCss >> result.txt 2>&1
+ ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+ CategoryInfo : NotSpecified: (C:\MyProject\...yl\index.js:142:String) [], RemoteException
+ FullyQualifiedErrorId : NativeCommandError
if (!isBuffer(val) && !isStream(val) && !isNull(val)) {
^
TypeError: isStream is not a function
at File.Object.defineProperty.set (C:\MyProject\node_modules\gulp\node_modul
es\vinyl-fs\node_modules\vinyl\index.js:142:28)
at new File (C:\MyProject\node_modules\gulp\node_modules\vinyl-fs\node_modul
es\vinyl\index.js:27:17)
at DestroyableTransform.createFile
(C:\MyProject\node_modules\gulp\node_modules\vinyl-fs\lib\src\index.js:12:12)
at DestroyableTransform.Transform._read (C:\MyProject\node_modules\gulp\node
_modules\readable-stream\lib\_stream_transform.js:184:10)
at DestroyableTransform.Transform._write (C:\MyProject\node_modules\gulp\nod
e_modules\readable-stream\lib\_stream_transform.js:172:12)
at doWrite (C:\MyProject\node_modules\gulp\node_modules\readable-stream\lib\
_stream_writable.js:237:10)
at writeOrBuffer (C:\MyProject\node_modules\gulp\node_modules\readable-strea
m\lib\_stream_writable.js:227:5)
at DestroyableTransform.Writable.write (C:\MyProject\node_modules\gulp\node_
modules\readable-stream\lib\_stream_writable.js:194:11)
at Stream.ondata (stream.js:31:26)
at emitOne (events.js:77:13)
Can anyone suggest anything? Either ideas for what might be wrong or ideas for how to diagnose the issue...?

Answer 1

Since you seem to directly access files from the node_modules folder, the first thing I would try is to erase the entire node_modules folder and reinstall all dependencies in case you inadvertently fraked something up.
If that doesn't help the offending line is here. isStream comes from the ./lib/isStream module which does nothing more than export a function. How you can end up with a TypeError: isStream is not a function message in that context is a bit of a mystery to me.
The only thing I can think of that could be of help is to add some debugging output. In C:\MyProject\node_modules\gulp\node_modules\vinyl-fs\node_modules\vinyl\index.js add the following right before the offending if statement:
console.log(require('util').inspect(isStream, { showHidden: true, depth: null}));
That should at least tell us what type isStream is at the time the error is thrown.