Consistency — please skip one more line, add a comment block and a `g` prefix (I know it’s *very* old-school and I regret this choice, but consistency doesn’t harm).
```javascript
  /**
   * The device width (px) can be either detected or specified in a global
   * `config' object. It is required in case media queries are used on some
   * l10n items.
   */

  var gDeviceWidth;
  if (typeof config !== 'undefined' && 'DEVICE_WIDTH' in config) {
    gDeviceWidth = parseInt(config.DEVICE_WIDTH);
  } else {
    gDeviceWidth = parseInt(screen.width);
  }
```