function loadModuleImplementation()

in packages/metro-runtime/src/polyfills/require.js [337:451]
91 lines of code
20 McCabe index (conditional complexity)

function loadModuleImplementation(
  moduleId: ModuleID,
  module: ?ModuleDefinition,
): Exports {
  if (!module && moduleDefinersBySegmentID.length > 0) {
    const segmentId = definingSegmentByModuleID.get(moduleId) ?? 0;
    const definer = moduleDefinersBySegmentID[segmentId];
    if (definer != null) {
      definer(moduleId);
      module = modules[moduleId];
      definingSegmentByModuleID.delete(moduleId);
    }
  }

  const nativeRequire = global.nativeRequire;
  if (!module && nativeRequire) {
    const {segmentId, localId} = unpackModuleId(moduleId);
    nativeRequire(localId, segmentId);
    module = modules[moduleId];
  }

  if (!module) {
    throw unknownModuleError(moduleId);
  }

  if (module.hasError) {
    throw moduleThrewError(moduleId, module.error);
  }

  if (__DEV__) {
    var Systrace = requireSystrace();
    var Refresh = requireRefresh();
  }

  // We must optimistically mark module as initialized before running the
  // factory to keep any require cycles inside the factory from causing an
  // infinite require loop.
  module.isInitialized = true;

  const {factory, dependencyMap} = module;
  if (__DEV__) {
    initializingModuleIds.push(moduleId);
  }
  try {
    if (__DEV__) {
      // $FlowIgnore: we know that __DEV__ is const and `Systrace` exists
      Systrace.beginEvent('JS_require_' + (module.verboseName || moduleId));
    }

    const moduleObject: Module = module.publicModule;

    if (__DEV__) {
      moduleObject.hot = module.hot;

      var prevRefreshReg = global.$RefreshReg$;
      var prevRefreshSig = global.$RefreshSig$;
      if (Refresh != null) {
        const RefreshRuntime = Refresh;
        global.$RefreshReg$ = (type, id) => {
          RefreshRuntime.register(type, moduleId + ' ' + id);
        };
        global.$RefreshSig$ =
          RefreshRuntime.createSignatureFunctionForTransform;
      }
    }
    moduleObject.id = moduleId;

    // keep args in sync with with defineModuleCode in
    // metro/src/Resolver/index.js
    // and metro/src/ModuleGraph/worker.js
    factory(
      global,
      metroRequire,
      metroImportDefault,
      metroImportAll,
      moduleObject,
      moduleObject.exports,
      dependencyMap,
    );

    // avoid removing factory in DEV mode as it breaks HMR
    if (!__DEV__) {
      // $FlowFixMe: This is only sound because we never access `factory` again
      module.factory = undefined;
      module.dependencyMap = undefined;
    }

    if (__DEV__) {
      // $FlowIgnore: we know that __DEV__ is const and `Systrace` exists
      Systrace.endEvent();

      if (Refresh != null) {
        registerExportsForReactRefresh(Refresh, moduleObject.exports, moduleId);
      }
    }

    return moduleObject.exports;
  } catch (e) {
    module.hasError = true;
    module.error = e;
    module.isInitialized = false;
    module.publicModule.exports = undefined;
    throw e;
  } finally {
    if (__DEV__) {
      if (initializingModuleIds.pop() !== moduleId) {
        throw new Error(
          'initializingModuleIds is corrupt; something is terribly wrong',
        );
      }
      global.$RefreshReg$ = prevRefreshReg;
      global.$RefreshSig$ = prevRefreshSig;
    }
  }
}