How do you create an empty stream in Gulp?

Question

SSCCE:

gulp.task('foo', [], function() {
    var barFiles = getBarFiles();
    var bazFiles = getBazFiles();

    var barStream, bazStream;
    if (barFiles && barFiles.length > 0) {
        barStream = gulp.src(barFiles);
    }
    if (bazStream && bazStream.length > 0) {
        bazStream = gulp.src(bazStream);
    }
    return eventStream
        .merge(barStream, bazStream)
        .pipe(g.concat('bar-and-baz'))
        .pipe(gulp.dest('./foo-output/'));
});

getBarFiles() or getBazFiles() may return an empty array, which is not allowed by gulp.src(): Error: Invalid glob argument, hence the need to wrap the creation of the stream with an if condition.

So the question is, how do I create an empty stream, so that it can be merged with the other empty on non-empty stream?

Answer 1

How about using:

return gulp.src('.');

For newer Gulp its possible you will need to add allowEmpty: true option as follows:

return gulp.src('.', {allowEmpty: true});

As I assume it should just pass current directory in the stream with no actions on it. Works for me.

Answer 2

A function for creating an empty stream for use in gulp (gleaned from vinyl-fs source) is this:

var through2 = require('through2');

function createEmptyStream() {
    var pass = through2.obj();
    process.nextTick(pass.end.bind(pass));
    return pass;
}

If you make barFiles = createEmptyStream() in the example, it is functionally the same.

Current version of gulp (3.9.1) and vinyl-fs (0.3.0) on 2016-03-05 allows for an empty src array (from my testing). It uses something similar to the above to create an empty stream. Your example works (mostly as-is) in current versions:

var eventStream = require('event-stream');
var gulp = require('gulp');
var g = require('gulp-load-plugins')();

function getBarFiles() {
    return [
    ];
}

function getBazFiles() {
    return [
        'baz.txt'
    ];
}

gulp.task('foo', [], function() {
    var barFiles = getBarFiles();
    var bazFiles = getBazFiles();
    var barStream = gulp.src(barFiles);
    var bazStream = gulp.src(bazFiles);

    return eventStream
        .merge(barStream, bazStream)
        .pipe(g.concat('bar-and-baz.txt'))
        .pipe(gulp.dest('./'));
});

gulp.task('default', [ 'foo' ]);

The file bar-and-baz.txt is a concatenation of the contents of all files in the globs returned from those two functions. An empty list of globs is accepted.