My basic gulp file

treys-gulpfile.js

// npm install gulp

//---- TODO'S ----//
// TODO: smarter linting

// Include gulp
var gulp = require('gulp');

// Include Our Plugins
var jshint = require('gulp-jshint');
var sass = require('gulp-sass');
var uglify = require('gulp-uglify');
var rename = require('gulp-rename');
var minifyCss = require('gulp-minify-css');
var sourcemaps = require('gulp-sourcemaps');
var autoprefixer = require('gulp-autoprefixer');
var plumber = require('gulp-plumber');
var gutil = require('gulp-util');
var notify = require('gulp-notify');
var include = require('gulp-include');
var babel = require('gulp-babel');
var changed = require('gulp-changed');
var imagemin = require('gulp-imagemin');
var jpegOptim = require('imagemin-jpegoptim');
var pngQuant = require('imagemin-pngquant');


// File Path VARs
// These can be an array if you want to use multiple sources
var jsSrcDir = 'assets/src/js/' // source directory
var jsSrc = jsSrcDir + 'project.js' // source
var jsDestName = 'project.min.js' // destination file name
var jsDest = 'assets/dist/js/' // destination

var scssSrcDir = 'assets/src/scss/' // source directory
var scssSrc = scssSrcDir + 'project.scss' // source
var scssDestName = 'style.min.css' // destination file name
var scssDest = 'assets/dist/css/' // desitnation

var imageSrc = 'assets/src/images' // source
var imageDest = 'assets/dist/images' // desitnation


// Gulp-Notify Error Handler
var onError = function(err) {
  notify.onError({
    title: 'Gulp',
    subtitle: 'Error:',
    message: '<%= error.message %>',
    sound: 'Sosumi',
  })(err);

  this.emit('end'); // Reset Gulp Watch
};

var compileSass = function() {
  gulp
    .src(scssSrc) // this all gets organized into this file
    .pipe(plumber({ errorHandler: onError }))
    .pipe(sourcemaps.init()) // start making src maps
    .pipe(sass()) // compile scss
    .pipe(
      autoprefixer({
        // autoprefixer
        browsers: ['last 2 versions'],
        cascade: false,
      }),
    )
    .pipe(minifyCss()) // minify
    .pipe(rename(scssDestName)) // create file
    .pipe(notify('Compiled SCSS'))
    .pipe(sourcemaps.write('.')) // finish src maps
    .pipe(gulp.dest(scssDest));
};

var compileJs = function() {
  gulp
    .src(jsSrc) // get project.js
    .pipe(plumber({ errorHandler: onError }))
    .pipe(include({ hardFail: true })) // Hard fail on Errors please
    .pipe(
      babel({
        compact: false,
        presets: ['@babel/env'],
      }),
    )
    .pipe(notify('Transpiled ES6'))
    .pipe(sourcemaps.init()) // start src maps
    .pipe(rename(jsDestName)) // minify the concat file
    .pipe(uglify()) // minify
    .on('error', function(err) {
      gutil.log(gutil.colors.red('[Error]'), err.toString());
    }) // Error reporting as needed
    .pipe(notify('Compiled JS'))
    .pipe(gulp.dest(jsDest))
    .pipe(sourcemaps.write('.')) // finish src maps
    .pipe(gulp.dest(jsDest));
};

/**
 * Compress Images (jpeg, jpg, png, svg)
 * @param {String} src - Source files
 * @param {String} dist - Directory to output to
 */
var compressImages = function(src, dist) {
  var extensions = 'png,jpg,jpeg,svg';
  var imageQuality = 60;
  src += '/**/*.{' + extensions + '}';

  gulp
    .src(src)
    .pipe(changed(dist))
    .pipe(
      imagemin({
        verbose: true,
        svgoPlugins: [
          { cleanupIDs: false },
          { removeUselessDefs: false },
          { removeViewBox: true },
          { convertPathData: false },
          { margePaths: false },
          { removeXMLProcInst: false },
        ],
        use: [pngQuant({ quality: imageQuality }), jpegOptim({ max: imageQuality })],
      }),
    )
    .pipe(gulp.dest(dist));
};


// Lint Task
// note: really lame linting
gulp.task('lint', function() {
  return gulp
    .src(jsWatch)
    .pipe(jshint())
    .pipe(jshint.reporter('default'));
});

// Concatenate Specified Scripts, Minify JS, Source Map JS
// Note: Sometimes the JS loading order matters, this is to keep control of that.
gulp.task('js', function() {
  compileJs();
});

// Compile Sass, Generate Source Maps, Auto-Prefix, Minify CSS
gulp.task('scss', function() {
  compileSass();
});

/**
 * Task: smash
 * @Desc: Compress Image Files (jpeg, jpg, png, svg)
 */
gulp.task('smash', function() {
  compressImages(imageSrc, imageDest);
});

/**
 * Task: Build
 * @Desc: Compile JS/SCSS files
 */
gulp.task('build', function() {
  // Compile Theme Js/Scss
  compileSass();
  compileJs();
  compressImages();
});

// Watch Files For Changes
gulp.task('watch', function() {
  //gulp.watch('assets/src/js/*.js', ['lint']);
  gulp.watch(
    [
      jsSrcDir + '**/*.js', // Watch all the js files
      jsSrc, // our main file
      '!' + jsDest + '*.js', // DO NOT watch any minified files (this creates a loop)
      jsSrcDir + 'tagular-events.js' // Additional
    ],
    ['js'],
  );
  gulp.watch(
    [
      scssSrcDir + '**/*.scss',
      scssSrc
    ],
    ['scss']);
});

// Default Task
gulp.task('default', ['scss', 'js', 'watch']);

Updated with a more modern workflow.