gulpfile.js

const gulp          = require("gulp");
const sass          = require("gulp-sass");
const sassLint      = require('gulp-sass-lint');
const clean         = require('gulp-clean');

// set paths ...
const config = {
	scssPath: "src/scss",
	destPath: "static/stylesheets",
  assetPath: "static/govuk-frontend/assets"
}

// Delete our old css files
gulp.task('clean-css', function cleanCss () {
  return gulp.src('static/stylesheets/**/*')
    .pipe(clean());
});

// compile scss to CSS
gulp.task("scss", function compileSass () {
  return gulp.src( config.scssPath + '/*.scss')
  .pipe(sass({outputStyle: 'expanded',
    includePaths: [ 'src/govuk_frontend_toolkit/stylesheets',
      'src/govuk_template/assets/stylesheets',
      'src/govuk_elements/assets/sass',
      'src/govuk-frontend']})).on('error', sass.logError)
  .pipe(gulp.dest(config.destPath));
});

// check .scss files against .sass-lint.yml config
const lintSCSS = () => 
  gulp
    .src('src/scss/**/*.s+(a|c)ss')
    .pipe(sassLint({
      files: {},
      configFile: '.sass-lint.yml'
    }))
    .pipe(sassLint.format())
    .pipe(sassLint.failOnError());

// Watch src folder for changes
gulp.task("watch", function watchAssets () {
  gulp.watch("src/scss/**/*", gulp.series("scss"));
});

gulp.task("copy-assets", function copyAssets () {
  return gulp.src('src/govuk-frontend/assets/**/*')
    .pipe(gulp.dest(config.assetPath));
});

gulp.task("generate", gulp.series(lintSCSS, "clean-css", "copy-assets", "scss"));

// Set watch as default task
gulp.task("default", gulp.series("watch"));

exports.lintSCSS = lintSCSS;