JsCollectionOptimizer::optimize

public JsCollectionOptimizer::optimize(array $js_assets)

system.performance.stale_file_threshold 

public function optimize(array $js_assets) {
  // Group the assets.
  $js_groups = $this->grouper->group($js_assets);

  // Now optimize (concatenate, not minify) and dump each asset group, unless
  // that was already done, in which case it should appear in
  // system.js_cache_files.
  // Drupal contrib can override this default JS aggregator to keep the same
  // grouping, optimizing and dumping, but change the strategy that is used to
  // determine when the aggregate should be rebuilt (e.g. mtime, HTTPS …).
  $map = $this->state->get('system.js_cache_files') ? : array();
  $js_assets = array();
  foreach ($js_groups as $order => $js_group) {
    // We have to return a single asset, not a group of assets. It is now up
    // to one of the pieces of code in the switch statement below to set the
    // 'data' property to the appropriate value.
    $js_assets[$order] = $js_group;
    unset($js_assets[$order]['items']);

    switch ($js_group['type']) {
      case 'file':
        // No preprocessing, single JS asset: just use the existing URI.
        if (!$js_group['preprocess']) {
          $uri = $js_group['items'][0]['data'];
          $js_assets[$order]['data'] = $uri;
        }
        // Preprocess (aggregate), unless the aggregate file already exists.
        else {
          $key = $this->generateHash($js_group);
          $uri = '';
          if (isset($map[$key])) {
            $uri = $map[$key];
          }
          if (empty($uri) || !file_exists($uri)) {
            // Concatenate each asset within the group.
            $data = '';
            foreach ($js_group['items'] as $js_asset) {
              // Optimize this JS file, but only if it's not yet minified.
              if (isset($js_asset['minified']) && $js_asset['minified']) {
                $data .= file_get_contents($js_asset['data']);
              }
              else {
                $data .= $this->optimizer->optimize($js_asset);
              }
              // Append a ';' and a newline after each JS file to prevent them
              // from running together.
              $data .= ";\n";
            }
            // Remove unwanted JS code that cause issues.
            $data = $this->optimizer->clean($data);
            // Dump the optimized JS for this group into an aggregate file.
            $uri = $this->dumper->dump($data, 'js');
            // Set the URI for this group's aggregate file.
            $js_assets[$order]['data'] = $uri;
            // Persist the URI for this aggregate file.
            $map[$key] = $uri;
            $this->state->set('system.js_cache_files', $map);
          }
          else {
            // Use the persisted URI for the optimized JS file.
            $js_assets[$order]['data'] = $uri;
          }
          $js_assets[$order]['preprocessed'] = TRUE;
        }
        break;

      case 'external':
        // We don't do any aggregation and hence also no caching for external
        // JS assets.
        $uri = $js_group['items'][0]['data'];
        $js_assets[$order]['data'] = $uri;
        break;
    }
  }

  return $js_assets;
}