Arnold Rimmer Arnold Rimmer - 2 months ago 21
Javascript Question

Gulp error: watch task has to be a function

Here is my gulpfile:

// Modules & Plugins
var gulp = require('gulp');
var concat = require('gulp-concat');
var myth = require('gulp-myth');
var uglify = require('gulp-uglify');
var jshint = require('gulp-jshint');
var imagemin = require('gulp-imagemin');

// Styles Task
gulp.task('styles', function() {
return gulp.src('app/css/*.css')
.pipe(concat('all.css'))
.pipe(myth())
.pipe(gulp.dest('dist'));
});

// Scripts Task
gulp.task('scripts', function() {
return gulp.src('app/js/*.js')
.pipe(jshint())
.pipe(jshint.reporter('default'))
.pipe(concat('all.js'))
.pipe(uglify())
.pipe(gulp.dest('dist'));
});

// Images Task
gulp.task('images', function() {
return gulp.src('app/img/*')
.pipe(imagemin())
.pipe(gulp.dest('dist/img'));
});

// Watch Task
gulp.task('watch', function() {
gulp.watch('app/css/*.css', 'styles');
gulp.watch('app/js/*.js', 'scripts');
gulp.watch('app/img/*', 'images');
});

// Default Task
gulp.task('default', gulp.parallel('styles', 'scripts', 'images', 'watch'));


If I run the
images
,
scripts
or
css
task alone it works. I had to add the
return
in the tasks - this wasn't in the book but googling showed me this was required.

The problem I have is that the
default
task errors:

[18:41:59] Error: watching app/css/*.css: watch task has to be a function (optionally generated by using gulp.parallel or gulp.series)
at Gulp.watch (/media/sf_VM_Shared_Dev/webdevadvlocal/gulp/public_html/gulp-book/node_modules/gulp/index.js:28:11)
at /media/sf_VM_Shared_Dev/webdevadvlocal/gulp/public_html/gulp-book/gulpfile.js:36:10
at taskWrapper (/media/sf_VM_Shared_Dev/webdevadvlocal/gulp/public_html/gulp-book/node_modules/undertaker/lib/set-task.js:13:15)
at bound (domain.js:287:14)
at runBound (domain.js:300:12)
at asyncRunner (/media/sf_VM_Shared_Dev/webdevadvlocal/gulp/public_html/gulp-book/node_modules/async-done/index.js:36:18)
at nextTickCallbackWith0Args (node.js:419:9)
at process._tickCallback (node.js:348:13)
at Function.Module.runMain (module.js:444:11)
at startup (node.js:136:18)


I think it is because there is also no
return
in the watch task. Also the error message isn't clear - at least to me. I tried adding a
return
after the last
gulp.watch()
but that didn't work either.

Answer

In gulp 3.x you could just pass the name of a task to gulp.watch() like this:

gulp.task('watch', function() {
  gulp.watch('app/css/*.css', ['styles']);
  gulp.watch('app/js/*.js', ['scripts']);
  gulp.watch('app/img/*', ['images']);
});

In gulp 4.x this is no longer the case. You have to pass a function. The customary way of doing this in gulp 4.x is to pass a gulp.series() invocation with only one task name. This returns a function that only executes the specified task:

gulp.task('watch', function() {
  gulp.watch('app/css/*.css', gulp.series('styles'));
  gulp.watch('app/js/*.js', gulp.series('scripts'));
  gulp.watch('app/img/*', gulp.series('images'));
});
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