implement solution as proposed in stryker-mutator#3480

packages/jest-runner/src/config-loaders/custom-jest-config-loader.ts

@@ -10,26 +10,32 @@ import type { requireResolve } from '@stryker-mutator/util';
 import { JestRunnerOptionsWithStrykerOptions } from '../jest-runner-options-with-stryker-options.js';
 import * as pluginTokens from '../plugin-tokens.js';
 
-import { JestConfigLoader } from './jest-config-loader.js';
+import { JestConfigLoaderAsync } from './jest-config-loader-async.js';
 
 /**
  * The Default config loader will load the Jest configuration using the package.json in the package root
  */
-export class CustomJestConfigLoader implements JestConfigLoader {
+export class CustomJestConfigLoader implements JestConfigLoaderAsync {
   public static inject = tokens(commonTokens.logger, commonTokens.options, pluginTokens.requireFromCwd);
 
   constructor(private readonly log: Logger, private readonly options: StrykerOptions, private readonly requireFromCwd: typeof requireResolve) {}
 
-  public loadConfig(): Config.InitialOptions {
-    const jestConfig = this.readConfigFromJestConfigFile() ?? this.readConfigFromPackageJson() ?? {};
+  public async loadConfig(): Promise<Config.InitialOptions> {
+    const jestConfig = (await this.readConfigFromJestConfigFile()) ?? this.readConfigFromPackageJson() ?? {};
     this.log.debug('Final jest config: %s', jestConfig);
     return jestConfig;
   }
 
-  private readConfigFromJestConfigFile(): Config.InitialOptions | undefined {
+  private async readConfigFromJestConfigFile(): Promise<Config.InitialOptions | undefined> {
     const configFilePath = this.resolveJestConfigFilePath();
     if (configFilePath) {
-      const config = this.requireFromCwd(configFilePath) as Config.InitialOptions;
+      const potentialConfig: unknown = this.requireFromCwd(configFilePath);
+      let config;
+      if (typeof potentialConfig === 'function') {
+        config = (await potentialConfig()) as Config.InitialOptions;
+      } else {
+        config = potentialConfig as Config.InitialOptions;
+      }
       this.log.debug(`Read Jest config from ${configFilePath}`);
       this.setRootDir(config, configFilePath);
       return config;

packages/jest-runner/src/config-loaders/jest-config-loader-async.ts

@@ -0,0 +1,18 @@
+/**
+ * The ConfigLoader interface is used to load different kinds of configurations for Jest.
+ * Custom ConfigLoaders should implement this interface, the OptionsEditor will then be able to use it to load a Jest configuration.
+ *
+ * ConfigLoaders are typically used for projects that do not provide their configuration via the package.json file (e.g. React).
+ * The loaderConfig method will return a usable config for Jest to use.
+ */
+
+import { Config } from '@jest/types';
+
+export interface JestConfigLoaderAsync {
+  /*
+   * Load the JSON representation of a Jest Configuration.
+   *
+   * @return {JestConfiguration} a Promise with an object containing the Jest configuration.
+   */
+  loadConfig(): Promise<Config.InitialOptions>;
+}