Bundle and minify static assets in ASP.NET Core

By Scott Addie and David Pine

This article explains the benefits of applying bundling and minification, including how these features can be used with ASP.NET Core web apps.

What is bundling and minification

Bundling and minification are two distinct performance optimizations you can apply in a web app. Used together, bundling and minification improve performance by reducing the number of server requests and reducing the size of the requested static assets.

Bundling and minification primarily improve the first page request load time. Once a web page has been requested, the browser caches the static assets (JavaScript, CSS, and images). So, bundling and minification don't improve performance when requesting the same page, or pages, on the same site requesting the same assets. If the expires header isn't set correctly on the assets and if bundling and minification isn't used, the browser's freshness heuristics mark the assets stale after a few days. Additionally, the browser requires a validation request for each asset. In this case, bundling and minification provide a performance improvement even after the first page request.

Bundling

Bundling combines multiple files into a single file. Bundling reduces the number of server requests that are necessary to render a web asset, such as a web page. You can create any number of individual bundles specifically for CSS, JavaScript, etc. Fewer files mean fewer HTTP requests from the browser to the server or from the service providing your application. This results in improved first page load performance.

Minification

Minification removes unnecessary characters from code without altering functionality. The result is a significant size reduction in requested assets (such as CSS, images, and JavaScript files). Common side effects of minification include shortening variable names to one character and removing comments and unnecessary whitespace.

Consider the following JavaScript function:

AddAltToImg = function (imageTagAndImageID, imageContext) {
    ///<signature>
    ///<summary> Adds an alt tab to the image
    // </summary>
    //<param name="imgElement" type="String">The image selector.</param>
    //<param name="ContextForImage" type="String">The image context.</param>
    ///</signature>
    var imageElement = $(imageTagAndImageID, imageContext);
    imageElement.attr('alt', imageElement.attr('id').replace(/ID/, ''));
}

Minification reduces the function to the following:

AddAltToImg=function(t,a){var r=$(t,a);r.attr("alt",r.attr("id").replace(/ID/,""))};

In addition to removing the comments and unnecessary whitespace, the following parameter and variable names were renamed as follows:

Original Renamed
imageTagAndImageID t
imageContext a
imageElement r

Impact of bundling and minification

The following table outlines differences between individually loading assets and using bundling and minification:

Action With B/M Without B/M Change
File Requests 7 18 157%
KB Transferred 156 264.68 70%
Load Time (ms) 885 2360 167%

Browsers are fairly verbose regarding HTTP request headers. The total bytes sent metric saw a significant reduction when bundling. The load time shows a significant improvement, however this example ran locally. Greater performance gains are realized when using bundling and minification with assets transferred over a network.

Choose a bundling and minification strategy

ASP.NET Core is compatible with WebOptimizer, an open-source bundling and minification solution. For set up instructions and sample projects, see WebOptimizer. ASP.NET Core doesn't provide a native bundling and minification solution.

Third-party tools, such as Gulp and Webpack, provide workflow automation for bundling and minification, as well as linting and image optimization. By using design-time bundling and minification, the minified files are created prior to the app's deployment. Bundling and minifying before deployment provides the advantage of reduced server load. However, it's important to recognize that design-time bundling and minification increases build complexity and only works with static files.

Environment-based bundling and minification

As a best practice, the bundled and minified files of your app should be used in a production environment. During development, the original files make for easier debugging of the app.

Specify which files to include in your pages by using the Environment Tag Helper in your views. The Environment Tag Helper only renders its contents when running in specific environments.

The following environment tag renders the unprocessed CSS files when running in the Development environment:

<environment include="Development">
    <link rel="stylesheet" href="~/lib/bootstrap/dist/css/bootstrap.css" />
    <link rel="stylesheet" href="~/css/site.css" />
</environment>
<environment names="Staging,Production">
    <link rel="stylesheet" href="https://ajax.aspnetcdn.com/ajax/bootstrap/3.3.7/css/bootstrap.min.css"
          asp-fallback-href="~/lib/bootstrap/dist/css/bootstrap.min.css"
          asp-fallback-test-class="sr-only" asp-fallback-test-property="position" asp-fallback-test-value="absolute" />
    <link rel="stylesheet" href="~/css/site.min.css" asp-append-version="true" />
</environment>

The following environment tag renders the bundled and minified CSS files when running in an environment other than Development. For example, running in Production or Staging triggers the rendering of these stylesheets:

<environment exclude="Development">
    <link rel="stylesheet" href="https://ajax.aspnetcdn.com/ajax/bootstrap/3.3.7/css/bootstrap.min.css"
          asp-fallback-href="~/lib/bootstrap/dist/css/bootstrap.min.css"
          asp-fallback-test-class="sr-only" asp-fallback-test-property="position" asp-fallback-test-value="absolute" />
    <link rel="stylesheet" href="~/css/site.min.css" asp-append-version="true" />
</environment>
<environment names="Staging,Production">
    <link rel="stylesheet" href="https://ajax.aspnetcdn.com/ajax/bootstrap/3.3.7/css/bootstrap.min.css"
          asp-fallback-href="~/lib/bootstrap/dist/css/bootstrap.min.css"
          asp-fallback-test-class="sr-only" asp-fallback-test-property="position" asp-fallback-test-value="absolute" />
    <link rel="stylesheet" href="~/css/site.min.css" asp-append-version="true" />
</environment>

Additional resources