Enforcing performance budgets with webpack

Modern web applications no longer rely on a single huge JavaScript package. Research shows that larger packages increase memory usage and CPU load, especially on low- and mid-end mobile devices. Webpack provides multiple features to reduce package size and control resource loading priorities, the most important of which are code segmentation and performance hints. Code segmentation divides the code into multiple packages, loading on demand or in parallel; performance tips detect whether the package size exceeds the specified threshold when building, in order to optimize or delete unnecessary code.

Webpack By default, warnings are issued when the resource size or entry point exceeds 250KB (244KiB), but you can configure how and size thresholds of performance prompts are displayed via performance object in the webpack.config.js file. This article will describe how to use this feature as the first line of defense to prevent performance degradation.

Customize your budget

The default resource and entry point size thresholds may not always match your needs, but can be configured. For example, a small blog may have a budget of 50KB (48.8KiB) each of the resources and entry points. The relevant settings in webpack.config.js are as follows:

 module.exports = {
  performance: {
    maxAssetSize: 50000,
    maxEntrypointSize: 50000,
  }
};

maxAssetSize and maxEntrypointSize properties control the threshold size (in bytes) of the resource and entry point, respectively. maxEntrypointSize ensures that packages created from files listed in entry object (usually JavaScript or Sass files) do not exceed the specified threshold; maxAssetSize enforces the same restrictions on other resources generated by Webpack (such as images, fonts, etc.).

An error appears when the threshold is exceeded

Webpack issues a warning when the budget threshold is exceeded by default. This is enough for the development environment, but not for the production environment. You can trigger an error by adding hints property to the performance object and setting it to 'error' :

 module.exports = {
  performance: {
    maxAssetSize: 50000,
    maxEntrypointSize: 50000,
    hints: 'error',
  }
};

Other valid values ​​for hints attribute are 'warning' and false . false will completely disable the warning, even if the specified limit is exceeded. It is not recommended to use false in production mode.

Exclude certain resources

Webpack enforces a size threshold for each type of resource it generates. If any resource exceeds the specified limit, an error will be thrown. For example, if Webpack is configured to process images, an error will occur as long as one of the images exceeds the threshold.

assetFilter property can be used to control the files used to calculate performance prompts:

 module.exports = {
  performance: {
    maxAssetSize: 50000,
    maxEntrypointSize: 50000,
    hints: 'error',
    assetFilter: function(assetFilename) {
      return !assetFilename.endsWith('.jpg');
    },
  }
};

This tells Webpack to exclude any files ending in .jpg when running performance prompt calculations. It can use more complex logic to meet the conditions of various environments, file types, and other resources.

limit

One current limitation is that the same budget threshold applies to all resources and entry points. In other words, multiple budgets are not yet set as needed, such as setting different restrictions for JavaScript, CSS, and image files. However, there are already open pull requests that should remove this restriction.

Summarize

It is useful to set up and enforce performance budgets at the beginning of the project. It will remind you to pay attention to the size of your dependencies and encourage you to find lighter alternatives to avoid over budget.

But the performance budget doesn’t stop there! Resource size is just one of many factors that affect performance, so more work is still needed to ensure the best experience is provided. Running Lighthouse tests is a great way to understand other metrics and suggestions for improvement.

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