How to hint an asynchronous operation with a JavaScript promise?

Question

For learning purposes, I want to show how promises can help solving time dependencies of database operations in JavaScript.

So, I want to show that:

db.find(14);

actually consumes time behind the scenes.

How could I illustrate this time dependency and a possible solution that a Promises provides?

What I have so far is this:

  // data store operation take time
  function _findByUsername(username) {
    var user = _.findWhere(Users, {username: username});
    if (!user) {
      Promise.reject(new Error("User not found."));
    }
    return Promise.resolve(user);
  }

possible duplicate of [Is it useful to always return a promise](http://stackoverflow.com/questions/22473728/is-it-useful-to-always-return-a-promise) — Benjamin Gruenbaum, Apr 24 '14 at 09:39
Related reading: http://blog.izs.me/post/59142742143/designing-apis-for-asynchrony — Benjamin Gruenbaum, Apr 24 '14 at 09:41
the `db.find()` should be an Async operation, e.g. like https://github.com/mulderp/echojs/blob/master/api/users.js#L109-L111 — poseid, Apr 24 '14 at 10:16

poseid · Accepted Answer · 2014-04-24T14:45:52.890

0

An option might be to use the delay function of Bluebird and add a comment as follows:

findByUsername(username) { 
   /* simulates the behavior of a database operation */
   return Promise.delay(10).thenReturn(_.findWhere(Users, {username: username}))
 }

edited Apr 24 '14 at 14:45

answered Apr 24 '14 at 10:19

poseid

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Why would you deliberately slow your code down? Is this just for mocking? – Benjamin Gruenbaum Apr 24 '14 at 11:33
yes. By adding a delay, one could simulate the influence of time at higher abstraction levels, in worst case, even the influence of time-outs. what do you think? the point here is to leave a database connection out, but understand influence of time while promise paths are evaluated. – poseid Apr 24 '14 at 12:00
I think the only correct way to determine the response times is to use a database and profile it. Everything other than profiling isn't very useful. – Benjamin Gruenbaum Apr 24 '14 at 12:01

How to hint an asynchronous operation with a JavaScript promise?

1 Answers1