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


var paths = {
 ...
 js: ['./js/**/*.js'], // whatever directory your JS files are
 dest: ['./dest/'] // whatever directory you'd like the concat'd file to be placed in
 ...
}


gulp.task('scripts', function() {
 return gulp.src(paths.js)
  .pipe(concat('app.js'))
  .pipe(gulp.dest(paths.dest));
});

gulp.task('watch', function() {
 ...
 gulp.watch(paths.js, ['scripts'];
 ...
});