install extensions via npm
npm install popper.js --save

//Require dependencies

    var
        gulp = require('gulp'),
        sass = require('gulp-sass'),
        sourcemaps = require('gulp-sourcemaps'),
        concat = require('gulp-concat'),
        uglify = require('gulp-uglify'),
        cleanCSS = require('gulp-clean-css'),
        autoprefixer = require('gulp-autoprefixer'),
        replace = require('gulp-replace')
    ;

//Define directories

    var
        buildDir = 'assets/build'
    ;

//Define paths

    var paths = {
        scripts: [
            'node_modules/jquery/dist/jquery.js',
            'node_modules/popper.js/dist/umd/popper.min.js',
            'node_modules/bootstrap/dist/js/bootstrap.js',
        ],
        scss: [
            'assets/scss/style.scss'
        ]
    };

// Compile SCSS into CSS and merge Fontello
gulp.task('sass', function () {
    return gulp.src(paths.scss)
        .pipe(sourcemaps.init())
        .pipe(sass().on('error', sass.logError))
        .pipe(autoprefixer({
            browsers: ['ie 10', '> 1%', 'Firefox <= 20']
        }))
        .pipe(cleanCSS({rebase: false}))
        .pipe(concat('style.min.css'))
        .pipe(sourcemaps.write('maps/'))
        .pipe(gulp.dest(buildDir + '/css'))
    ;
});

// Minify and merge all JavaScript, compile source maps
gulp.task('scripts', function () {
    return gulp.src(paths.scripts)
        .pipe(sourcemaps.init())
        .pipe(uglify())
        .pipe(concat('all.min.js'))
        .pipe(sourcemaps.write('maps/'))
        .pipe(gulp.dest(buildDir + '/js'))
    ;
});


//Watch for changes to files and execute relevant tasks
gulp.task('watch', function (){
    gulp.watch(['assets/scss/style.scss', 'assets/scss/**/*.scss'], ['sass']);
    gulp.watch(paths.scripts, ['scripts']);
});

gulp.task('default', ['sass','scripts']);