next.js/packages/next/src/build/webpack/loaders/next-style-loader/index.ts

import path from 'path'
import { stringifyRequest } from '../../stringify-request'

const loaderApi = () => {}

loaderApi.pitch = function loader(this: any, request: any): any {
  const loaderSpan = this.currentTraceSpan.traceChild('next-style-loader')

  return loaderSpan.traceFn(() => {
    const options = this.getOptions()

    const insert =
      typeof options.insert === 'undefined'
        ? '"head"'
        : typeof options.insert === 'string'
          ? JSON.stringify(options.insert)
          : options.insert.toString()
    const injectType = options.injectType || 'styleTag'
    const esModule =
      typeof options.esModule !== 'undefined' ? options.esModule : false

    delete options.esModule

    switch (injectType) {
      case 'linkTag': {
        const hmrCode = this.hot
          ? `
if (module.hot) {
  module.hot.accept(
    ${stringifyRequest(this, `!!${request}`)},
    function() {
     ${
       esModule
         ? 'update(content);'
         : `content = require(${stringifyRequest(this, `!!${request}`)});

           content = content.__esModule ? content.default : content;

           update(content);`
     }
    }
  );

  module.hot.dispose(function() {
    update();
  });
}`
          : ''

        return `${
          esModule
            ? `import api from ${stringifyRequest(
                this,
                `!${path.join(__dirname, 'runtime/injectStylesIntoLinkTag.js')}`
              )};
            import content from ${stringifyRequest(this, `!!${request}`)};`
            : `var api = require(${stringifyRequest(
                this,
                `!${path.join(__dirname, 'runtime/injectStylesIntoLinkTag.js')}`
              )});
            var content = require(${stringifyRequest(this, `!!${request}`)});

            content = content.__esModule ? content.default : content;`
        }

var options = ${JSON.stringify(options)};

options.insert = ${insert};

var update = api(content, options);

${hmrCode}

${esModule ? 'export default {}' : ''}`
      }

      case 'lazyStyleTag':
      case 'lazySingletonStyleTag': {
        const isSingleton = injectType === 'lazySingletonStyleTag'

        const hmrCode = this.hot
          ? `
if (module.hot) {
  if (!content.locals || module.hot.invalidate) {
    
    var isEqualLocals = ${
      // eslint-disable-next-line @next/internal/typechecked-require -- Not a module.
      require('./runtime/isEqualLocals').toString()
    };
    console.log({isEqualLocals})
    var oldLocals = content.locals;

    module.hot.accept(
      ${stringifyRequest(this, `!!${request}`)},
      function () {
        ${
          esModule
            ? `if (!isEqualLocals(oldLocals, content.locals)) {
                module.hot.invalidate();

                return;
              }

              oldLocals = content.locals;

              if (update && refs > 0) {
                update(content);
              }`
            : `content = require(${stringifyRequest(this, `!!${request}`)});

              content = content.__esModule ? content.default : content;

              if (!isEqualLocals(oldLocals, content.locals)) {
                module.hot.invalidate();

                return;
              }

              oldLocals = content.locals;

              if (update && refs > 0) {
                update(content);
              }`
        }
      }
    )
  }

  module.hot.dispose(function() {
    if (update) {
      update();
    }
  });
}`
          : ''

        return `${
          esModule
            ? `import api from ${stringifyRequest(
                this,
                `!${path.join(
                  __dirname,
                  'runtime/injectStylesIntoStyleTag.js'
                )}`
              )};
            import content from ${stringifyRequest(this, `!!${request}`)};`
            : `var api = require(${stringifyRequest(
                this,
                `!${path.join(
                  __dirname,
                  'runtime/injectStylesIntoStyleTag.js'
                )}`
              )});
            var content = require(${stringifyRequest(this, `!!${request}`)});

            content = content.__esModule ? content.default : content;

            if (typeof content === 'string') {
              content = [[module.id, content, '']];
            }`
        }

var refs = 0;
var update;
var options = ${JSON.stringify(options)};

options.insert = ${insert};
options.singleton = ${isSingleton};

var exported = {};

exported.locals = content.locals || {};
exported.use = function() {
  if (!(refs++)) {
    update = api(content, options);
  }

  return exported;
};
exported.unuse = function() {
  if (refs > 0 && !--refs) {
    update();
    update = null;
  }
};

${hmrCode}

${esModule ? 'export default' : 'module.exports ='} exported;`
      }

      case 'styleTag':
      case 'singletonStyleTag':
      default: {
        const isSingleton = injectType === 'singletonStyleTag'

        const hmrCode = this.hot
          ? `
if (module.hot) {
  if (!content.locals || module.hot.invalidate) {
    var isEqualLocals = ${
      // eslint-disable-next-line @next/internal/typechecked-require -- Not a module.
      require('./runtime/isEqualLocals').toString()
    };
    var oldLocals = content.locals;

    module.hot.accept(
      ${stringifyRequest(this, `!!${request}`)},
      function () {
        ${
          esModule
            ? `if (!isEqualLocals(oldLocals, content.locals)) {
                module.hot.invalidate();

                return;
              }

              oldLocals = content.locals;

              update(content);`
            : `content = require(${stringifyRequest(this, `!!${request}`)});

              content = content.__esModule ? content.default : content;

              if (typeof content === 'string') {
                content = [[module.id, content, '']];
              }

              if (!isEqualLocals(oldLocals, content.locals)) {
                module.hot.invalidate();

                return;
              }

              oldLocals = content.locals;

              update(content);`
        }
      }
    )
  }

  module.hot.dispose(function() {
    update();
  });
}`
          : ''

        return `${
          esModule
            ? `import api from ${stringifyRequest(
                this,
                `!${path.join(
                  __dirname,
                  'runtime/injectStylesIntoStyleTag.js'
                )}`
              )};
            import content from ${stringifyRequest(this, `!!${request}`)};`
            : `var api = require(${stringifyRequest(
                this,
                `!${path.join(
                  __dirname,
                  'runtime/injectStylesIntoStyleTag.js'
                )}`
              )});
            var content = require(${stringifyRequest(this, `!!${request}`)});

            content = content.__esModule ? content.default : content;

            if (typeof content === 'string') {
              content = [[module.id, content, '']];
            }`
        }

var options = ${JSON.stringify(options)};

options.insert = ${insert};
options.singleton = ${isSingleton};

var update = api(content, options);

${hmrCode}

${esModule ? 'export default' : 'module.exports ='} content.locals || {};`
      }
    }
  })
}

module.exports = loaderApi