promise & . then

http://stackoverflow.com/questions/3884281/what-does-the-function-then-mean-in-javascript

The traditional way to deal with asynchronous calls in JavaScript has been with callbacks. Say we had to make three calls to the server, one after the other, to set up our application. With callbacks, the code might look something like the following (assuming a xhrGET function to make the server call):

// Fetch some server configuration
    xhrGET('/api/server-config', function(config) {
        // Fetch the user information, if he's logged in
        xhrGET('/api/' + config.USER_END_POINT, function(user) {
            // Fetch the items for the user
            xhrGET('/api/' + user.id + '/items', function(items) {
                // Actually display the items here
            });
        });
    });

In this example, we first fetch the server configuration. Then based on that, we fetch information about the current user, and then finally get the list of items for the current user. Each xhrGET call takes a callback function that is executed when the server responds.

Now of course the more levels of nesting we have, the harder the code is to read, debug, maintain, upgrade, and basically work with. This is generally known as callback hell. Also, if we needed to handle errors, we need to possibly pass in another function to each xhrGET call to tell it what it needs to do in case of an error. If we wanted to have just one common error handler, that is not possible.

The Promise API was designed to solve this nesting problem and the problem of error handling.

The Promise API proposes the following:

  1. Each asynchronous task will return a promise object.
  2. Each promise object will have a then function that can take two arguments, a successhandler and an error handler.
  3. The success or the error handler in the then function will be called only once, after the asynchronous task finishes.
  4. The then function will also return a promise, to allow chaining multiple calls.
  5. Each handler (success or error) can return a value, which will be passed to the next function as an argument, in the chain of promises.
  6. If a handler returns a promise (makes another asynchronous request), then the next handler (success or error) will be called only after that request is finished.
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