Method onChange

packages/jest-haste-map/src/index.ts:875–1012

(
      type: string,
      filePath: string,
      root: string,
      stat?: Stats,
    )

    };

    const onChange = (
      type: string,
      filePath: string,
      root: string,
      stat?: Stats,
    ) => {
      filePath = path.join(root, normalizePathSep(filePath));
      if (
        (stat && stat.isDirectory()) ||
        this._ignore(filePath) ||
        !extensions.some(extension => filePath.endsWith(extension))
      ) {
        return;
      }

      const relativeFilePath = fastPath.relative(rootDir, filePath);
      const fileMetadata = hasteMap.files.get(relativeFilePath);

      // The file has been accessed, not modified
      if (
        type === 'change' &&
        fileMetadata &&
        stat &&
        fileMetadata[H.MTIME] === stat.mtime.getTime()
      ) {
        return;
      }

      changeQueue = changeQueue
        .then(() => {
          // If we get duplicate events for the same file, ignore them.
          if (
            eventsQueue.some(
              event =>
                event.type === type &&
                event.filePath === filePath &&
                ((!event.stat && !stat) ||
                  (!!event.stat &&
                    !!stat &&
                    event.stat.mtime.getTime() === stat.mtime.getTime())),
            )
          ) {
            return null;
          }

          if (mustCopy) {
            mustCopy = false;
            hasteMap = {
              clocks: new Map(hasteMap.clocks),
              duplicates: new Map(hasteMap.duplicates),
              files: new Map(hasteMap.files),
              map: new Map(hasteMap.map),
              mocks: new Map(hasteMap.mocks),
            };
          }

          const add = () => {
            eventsQueue.push({filePath, stat, type});