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svelte-loader

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A webpack loader for svelte.

Install

npm install --save svelte svelte-loader

Usage

Configure inside your webpack.config.js:

  ...
  resolve: {
    // see below for an explanation
    alias: {
      svelte: path.resolve('node_modules', 'svelte')
    },
    extensions: ['.mjs', '.js', '.svelte'],
    mainFields: ['svelte', 'browser', 'module', 'main']
  },
  module: {
    rules: [
      ...
      {
        test: /\.(html|svelte)$/,
        exclude: /node_modules/,
        use: 'svelte-loader'
      },
      {
        // required to prevent errors from Svelte on Webpack 5+, omit on Webpack 4
        test: /node_modules\/svelte\/.*\.mjs$/,
        resolve: {
          fullySpecified: false
        }
      }
      ...
    ]
  }
  ...

Check out the example project.

resolve.alias

The resolve.alias option is used to make sure that only one copy of the Svelte runtime is bundled in the app, even if you are npm linking in dependencies with their own copy of the svelte package. Having multiple copies of the internal scheduler in an app, besides being inefficient, can also cause various problems.

resolve.mainFields

Webpack's resolve.mainFields option determines which fields in package.json are used to resolve identifiers. If you're using Svelte components installed from npm, you should specify this option so that your app can use the original component source code, rather than consuming the already-compiled version (which is less efficient).

Extracting CSS

If your Svelte components contain <style> tags, by default the compiler will add JavaScript that injects those styles into the page when the component is rendered. That's not ideal, because it adds weight to your JavaScript, prevents styles from being fetched in parallel with your code, and can even cause CSP violations.

A better option is to extract the CSS into a separate file. Using the emitCss option as shown below would cause a virtual CSS file to be emitted for each Svelte component. The resulting file is then imported by the component, thus following the standard Webpack compilation flow. Add MiniCssExtractPlugin to the mix to output the css to a separate file.

const MiniCssExtractPlugin = require('mini-css-extract-plugin');
const mode = process.env.NODE_ENV || 'development';
const prod = mode === 'production';
  ...
  module: {
    rules: [
      ...
      {
        test: /\.(html|svelte)$/,
        exclude: /node_modules/,
        use: {
          loader: 'svelte-loader',
          options: {
            emitCss: true,
          },
        },
      },
      {
        test: /\.css$/,
        use: [
          prod ? MiniCssExtractPlugin.loader :'style-loader',
          {
            loader: 'css-loader',
            options: {
              url: false, //necessary if you use url('/path/to/some/asset.png|jpg|gif')
            }
          }
        ]
      },
      ...
    ]
  },
  ...
  plugins: [
    new MiniCssExtractPlugin('styles.css'),
    ...
  ]
  ...

Note that the configuration shown above switches off MiniCssExtractPlugin in development mode in favour of using CSS javascript injection. This is recommended by MiniCssExtractPlugin because it does not support hot reloading.

prod indicates, that NODE_ENV=production has been set from package.json or manually (NODE_ENV=production npx webpack) for production builds. We can rely on that to make dynamic adjustments to the config.

Additionally, if you're using multiple entrypoints, you may wish to change new MiniCssExtractPlugin('styles.css') for new MiniCssExtractPlugin('[name].css') to generate one CSS file per entrypoint.

Warning: in production, if you have set sideEffects: false in your package.json, MiniCssExtractPlugin has a tendency to drop CSS, regardless of whether it's included in your svelte components.

Alternatively, if you're handling styles in some other way and just want to prevent the CSS being added to your JavaScript bundle, use

...
use: {
  loader: 'svelte-loader',
  options: {
    compilerOptions: {
      css: false
    }
  },
},
...

Source maps

JavaScript source maps are enabled by default, you just have to use an appropriate webpack devtool.

To enable CSS source maps, you'll need to use emitCss and pass the sourceMap option to the css-loader. The above config should look like this:

module.exports = {
    ...
    devtool: "source-map", // any "source-map"-like devtool is possible
    ...
    module: {
      rules: [
        ...
        {
          test: /\.css$/,
          use: [
            prod ? MiniCssExtractPlugin.loader :'style-loader',
            {
              loader: 'css-loader',
              options: {
                sourceMap: true
              }
            }
          ]
        },
        ...
      ]
    },
    ...
    plugins: [
      new MiniCssExtractPlugin('styles.css'),
      ...
    ]
    ...
};

This should create an additional styles.css.map file.

Svelte Compiler options

You can specify additional arbitrary compilation options with the compilerOptions config key, which are passed directly to the underlying Svelte compiler:

...
use: {
  loader: 'svelte-loader',
  options: {
    compilerOptions: {
      // additional compiler options here
      generate: 'ssr', // for example, SSR can be enabled here
    }
  },
},
...

Hot Reload

This loader supports component-level HMR via the community supported svelte-hmr package. This package serves as a testbed and early access for Svelte HMR, while we figure out how to best include HMR support in the compiler itself (which is tricky to do without unfairly favoring any particular dev tooling). Feedback, suggestion, or help to move HMR forward is welcomed at svelte-hmr (for now).

Configure inside your webpack.config.js:

// It is recommended to adjust svelte options dynamically, by using
// environment variables
const mode = process.env.NODE_ENV || 'development';
const prod = mode === 'production';

module.exports = {
  ...
  module: {
    rules: [
      ...
      {
        test: /\.(html|svelte)$/,
        exclude: /node_modules/,
        use: {
          loader: 'svelte-loader',
          options: {
            compilerOptions: {
              // NOTE Svelte's dev mode MUST be enabled for HMR to work
              dev: !prod, // Default: false
            },

            // NOTE emitCss: true is currently not supported with HMR
            // Enable it for production to output separate css file
            emitCss: prod, // Default: false
            // Enable HMR only for dev mode
            hotReload: !prod, // Default: false
            // Extra HMR options, the defaults are completely fine
            // You can safely omit hotOptions altogether
            hotOptions: {
              // Prevent preserving local component state
              preserveLocalState: false,

              // If this string appears anywhere in your component's code, then local
              // state won't be preserved, even when noPreserveState is false
              noPreserveStateKey: '@!hmr',

              // Prevent doing a full reload on next HMR update after fatal error
              noReload: false,

              // Try to recover after runtime errors in component init
              optimistic: false,

              // --- Advanced ---

              // Prevent adding an HMR accept handler to components with
              // accessors option to true, or to components with named exports
              // (from <script context="module">). This have the effect of
              // recreating the consumer of those components, instead of the
              // component themselves, on HMR updates. This might be needed to
              // reflect changes to accessors / named exports in the parents,
              // depending on how you use them.
              acceptAccessors: true,
              acceptNamedExports: true,
            }
          }
        }
      }
      ...
    ]
  },
  plugins: [
    new webpack.HotModuleReplacementPlugin(),
    ...
  ]
}

You also need to add the HotModuleReplacementPlugin. There are multiple ways to achieve this.

If you're using webpack-dev-server, you can just pass it the hot option to add the plugin automatically.

Otherwise, you can add it to your webpack config directly:

const webpack = require('webpack');

module.exports = {
  ...
  plugins: [
    new webpack.HotModuleReplacementPlugin(),
    ...
  ]
}

License

MIT