expressjs.com/en/api/app-configure.jade

section
  h3(id='app.configure') app.configure([env], callback)

  p.
    Conditionally invoke <code>callback</code> when <code>env</code> matches <code>app.get('env')</code>,
    aka <code>process.env.NODE_ENV</code>. This method remains for legacy reason, and is effectively
    an <code>if</code> statement as illustrated in the following snippets. These functions are <em>not</em>
    required in order to use <code>app.set()</code> and other configuration methods.

  +js.
    // all environments
    app.configure(function(){
      app.set('title', 'My Application');
    })

    // development only
    app.configure('development', function(){
      app.set('db uri', 'localhost/dev');
    })
    
    // production only
    app.configure('production', function(){
      app.set('db uri', 'n.n.n.n/prod');
    })

  Is effectively sugar for:

  +js.
    // all environments
    app.set('title', 'My Application');

    // development only
    if ('development' == app.get('env')) {
      app.set('db uri', 'localhost/dev');
    }
    
    // production only
    if ('production' == app.get('env')) {
      app.set('db uri', 'n.n.n.n/prod');
    }