code: function isChildCompilerCacheOutdated (mainCompilation, childCompiler) {
    // If the compilation was never run there is no invalid cache
    if (!childCompiler.compilationStartedTimestamp) {
      return false;
    }
    // Check if any dependent file was changed after the last compilation
    const fileTimestamps = mainCompilation.fileTimestamps;
    const isCacheOutOfDate = childCompiler.fileDependencies.some((fileDependency) => {
      const timestamp = fileTimestamps.get(fileDependency);
      // If the timestamp is not known the file is new
      // If the timestamp is larger then the file has changed
      // Otherwise the file is still the same
      return !timestamp || timestamp > childCompiler.compilationStartedTimestamp;
    });
    return isCacheOutOfDate;
  }
docs: Returns `true` if the file dependencies of the given childCompiler are outdated.
  
  @param {WebpackCompilation} mainCompilation
  @param {HtmlWebpackChildCompiler} childCompiler
  @returns {boolean}