@backstage/cli/config/jestCachingModuleLoader.js

/*
 * Copyright 2022 The Backstage Authors
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *     http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

const fs = require('fs');
const { default: JestRuntime } = require('jest-runtime');

const fileTransformCache = new Map();
const scriptTransformCache = new Map();

let runtimeGeneration = 0;

module.exports = class CachingJestRuntime extends JestRuntime {
  // Each Jest run creates a new runtime, including when rerunning tests in
  // watch mode. This keeps track of whether we've switched runtime instance.
  __runtimeGeneration = runtimeGeneration++;

  transformFile(filename, options) {
    const entry = fileTransformCache.get(filename);
    if (entry) {
      // Only check modification time if it's from a different runtime generation
      if (entry.generation === this.__runtimeGeneration) {
        return entry.code;
      }

      // Keep track of the modification time of files so that we can properly
      // reprocess them in watch mode.
      const { mtimeMs } = fs.statSync(filename);
      if (mtimeMs > entry.mtimeMs) {
        const code = super.transformFile(filename, options);
        fileTransformCache.set(filename, {
          code,
          mtimeMs,
          generation: this.__runtimeGeneration,
        });
        return code;
      }

      fileTransformCache.set(filename, {
        ...entry,
        generation: this.__runtimeGeneration,
      });
      return entry.code;
    }

    const code = super.transformFile(filename, options);
    fileTransformCache.set(filename, {
      code,
      mtimeMs: fs.statSync(filename).mtimeMs,
      generation: this.__runtimeGeneration,
    });
    return code;
  }

  // This may or may not be a good idea. Theoretically I don't know why this would impact
  // test correctness and flakiness, but it seems like it may introduce flakiness and strange failures.
  // It does seem to speed up test execution by a fair amount though.
  createScriptFromCode(scriptSource, filename) {
    let script = scriptTransformCache.get(scriptSource);
    if (!script) {
      script = super.createScriptFromCode(scriptSource, filename);
      // Tried to store the script object in a WeakRef here. It starts out at
      // about 90% hit rate, but eventually drops all the way to 20%, and overall
      // it seemed to increase memory usage by 20% or so.
      scriptTransformCache.set(scriptSource, script);
    }
    return script;
  }
};