Conditionally load module and set JS config var

conditionally-add-js-var.php

/* declare your module */

$wgResourceModules['ext.Foo'] = array(
  'scripts' => array( 'js/ext.Foo.js' ),
  'localBasePath' => dirname( __FILE__ ),
  'remoteExtPath' => 'MyExtension'
);


/* hook into BeforePageDisplay */

$wgHooks['BeforePageDisplay'][] = 'loadConditionally';


/* load module and set config var if some condition is true */

function loadConditionally( &$out ) {
  if ( someCondition() ) {
    $out->addJsConfigVars( array (
      "wgAnswer" => 42,
      "wgServerDate" => getdate();
    ) );
    $out->addModules( 'ext.Foo' );
  }
  return true;
}