You can do something like this (using Bluebird's Promise.delay for convenience):

    // using Promise.race so whoever finishes first wins
    var fetchingWithTimeout = Promise.race([
      
      // first promise: actually try to perform the computation
      fetching,
      
      // second promise: wait x seconds, then reject
      Promise.delay(x*1000).then(Promise.reject('Timed out')),
      
    ]);

You can apply the timeout wherever you want (or apply different timeouts in different corners of the app) without throwing out the raw promise to perform the computation no matter the time cost.