Gulp 4: Passthrough source streams

September 7, 2015

Another nice addition to vinyl-fs that will end up in Gulp 4 is the possibility of having “passthrough” source streams. This basically allows gulp.src to be writable. So what does this mean for you?

Usually, gulp.src would create a stream of file objects based on the globbing pattern you provide. This made gulp.src mandatory to be at the beginning of your stream. Now it can be anywhere in your pipeline, carrying over the intermediate results from the earlier steps.

This makes up for some pretty interesting scenarios. The most useful one is definitely merging streams. See the example below: We want to lint our self-written JavaScript files, and concatenate them with vendor specific files to one single JavaScript file:

var gulp = require('gulp');
var concat = require('gulp-concat');
var jshint = require('gulp-jshint');
var uglify = require('gulp-uglify');

gulp.task('default', function() {
  return gulp.src('src/**/*.js') /** 1 **/
    .pipe(jshint()) /** 2 **/
    .pipe(jshint.reporter('default'))
    .pipe(jshint.reporter('fail'))
    .pipe(gulp.src('vendor/**/*.js', {passthrough: true})) /** 3 **/
    .pipe(uglify())
    .pipe(concat('main.min.js'))
    .pipe(gulp.dest('dest'));
});

The process explained:

We glob all our source JavaScript files.
Those are the files we create, which means we want to have them linted to our coding conventions
After the JSHint process, we get all our vendor files. Those include things like jQuery, lodash, you name it. By using the passthrough flag, all the file objects from the original pipeline are passed through, and thus added to the whole stream.

We can also benefit from this feature when we want to merge preprocessor output with plain source files:

gulp.task('styles', function(){
  return gulp.src('styles/main.scss')
    .pipe(sass())
    .pipe(gulp.src('styles/**/*.css'), {passthrough: true})
    .pipe(concat('main.css'))
    .pipe(gulp.dest('dist'));
});

Same with CoffeeScript, if you are into that:

gulp.task('scripts', function(){
  return gulp.src('scripts/*.coffee')
    .pipe(coffee())
    .pipe(gulp.src('scripts/*.js'), {passthrough: true})
    .pipe(uglify())
    .pipe(gulp.dest('dist'));
});

While this doesn’t solve all the scenarios where merging comes in handy, combining different sources mid-stream is definitely a welcomed addition.

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Gulp 4: Passthrough source streams

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