By [Erik Reitan](https://github.com/Erikre), [Scott Addie](https://scottaddie.com), [Daniel Roth](https://github.com/danroth27), and [Shayne Boyer](https://twitter.com/spboyer)
A *task runner* is a tool which automates these routine development tasks and more. Visual Studio provides built-in support for two popular JavaScript-based task runners: [Gulp](https://gulpjs.com/) and [Grunt](using-grunt.md).
Gulp is a JavaScript-based streaming build toolkit for client-side code. It's commonly used to stream client-side files through a series of processes when a specific event is triggered in a build environment. For instance, Gulp can be used to automate [bundling and minification](bundling-and-minification.md) or the cleansing of a development environment before a new build.
A set of Gulp tasks is defined in *gulpfile.js*. The following JavaScript includes Gulp modules and specifies file paths to be referenced within the forthcoming tasks:
The above code specifies which Node modules are required. The `require` function imports each module so that the dependent tasks can utilize their features. Each of the imported modules is assigned to a variable. The modules can be located either by name or path. In this example, the modules named `gulp`, `rimraf`, `gulp-concat`, `gulp-cssmin`, and `gulp-uglify` are retrieved by name. Additionally, a series of paths are created so that the locations of CSS and JavaScript files can be reused and referenced within the tasks. The following table provides descriptions of the modules included in *gulpfile.js*.
| gulp | The Gulp streaming build system. For more information, see [gulp](https://www.npmjs.com/package/gulp). |
| rimraf | A Node deletion module. For more information, see [rimraf](https://www.npmjs.com/package/rimraf). |
| gulp-concat | A module that concatenates files based on the operating system's newline character. For more information, see [gulp-concat](https://www.npmjs.com/package/gulp-concat). |
| gulp-cssmin | A module that minifies CSS files. For more information, see [gulp-cssmin](https://www.npmjs.com/package/gulp-cssmin). |
| gulp-uglify | A module that minifies *.js* files. For more information, see [gulp-uglify](https://www.npmjs.com/package/gulp-uglify). |
The bindings you set up with **Task Runner Explorer** are stored in the form of a comment at the top of your *gulpfile.js* and are effective only in Visual Studio. An alternative that doesn't require Visual Studio is to configure automatic execution of gulp tasks in your *.csproj* file. For example, put this in your *.csproj* file:
Now the clean task is executed when you run the project in Visual Studio or from a command prompt using the `dotnet run` command (run `npm install` first).
When you run multiple tasks, the tasks run concurrently by default. However, if you need to run tasks in a specific order, you must specify when each task is complete, as well as which tasks depend on the completion of another task.
1. To define a series of tasks to run in order, replace the `first` task that you added above in *gulpfile.js* with the following:
You now have three tasks: `series:first`, `series:second`, and `series`. The `series:second` task includes a second parameter which specifies an array of tasks to be run and completed before the `series:second` task will run. As specified in the code above, only the `series:first` task must be completed before the `series:second` task will run.
IntelliSense provides code completion, parameter descriptions, and other features to boost productivity and to decrease errors. Gulp tasks are written in JavaScript; therefore, IntelliSense can provide assistance while developing. As you work with JavaScript, IntelliSense lists the objects, functions, properties, and parameters that are available based on your current context. Select a coding option from the pop-up list provided by IntelliSense to complete the code.
When Gulp is used to optimize client-side files for staging and production, the processed files are saved to a local staging and production location. The *_Layout.cshtml* file uses the **environment** tag helper to provide two different versions of CSS files. One version of CSS files is for development and the other version is optimized for both staging and production. In Visual Studio 2017, when you change the **ASPNETCORE_ENVIRONMENT** environment variable to `Production`, Visual Studio will build the Web app and link to the minimized CSS files. The following markup shows the **environment** tag helpers containing link tags to the `Development` CSS files and the minified `Staging, Production` CSS files.
A Gulp task is registered with a function name. You can specify dependencies if other tasks must run before the current task. Additional functions allow you to run and watch the Gulp tasks, as well as set the source (*src*) and destination (*dest*) of the files being modified. The following are the primary Gulp API functions:
|task |`gulp.task(name[, deps], fn) { }`|The `task` function creates a task. The `name` parameter defines the name of the task. The `deps` parameter contains an array of tasks to be completed before this task runs. The `fn` parameter represents a callback function which performs the operations of the task.|
|watch |`gulp.watch(glob [, opts], tasks) { }`|The `watch` function monitors files and runs tasks when a file change occurs. The `glob` parameter is a `string` or `array` that determines which files to watch. The `opts` parameter provides additional file watching options.|
|src |`gulp.src(globs[, options]) { }`|The `src` function provides files that match the glob value(s). The `glob` parameter is a `string` or `array` that determines which files to read. The `options` parameter provides additional file options.|
|dest |`gulp.dest(path[, options]) { }`|The `dest` function defines a location to which files can be written. The `path` parameter is a string or function that determines the destination folder. The `options` parameter is an object that specifies output folder options.|
For additional Gulp API reference information, see [Gulp Docs API](https://github.com/gulpjs/gulp/blob/master/docs/API.md).
The Gulp community provides Gulp [recipes](https://github.com/gulpjs/gulp/blob/master/docs/recipes/README.md). These recipes consist of Gulp tasks to address common scenarios.
