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`catch` is simply sugar for `then(undefined, onRejection)` which makes it the same as the catch block of a try/catch statement. ```js function findAuthor(){ throw new Error('couldn't find that author'); } // synchronous try { findAuthor(); } catch(reason) { // something went wrong } // async with promises findAuthor().catch(function(reason){ // something went wrong }); ```

The primary way of interacting with a promise is through its `then` method, which registers callbacks to receive either a promise's eventual value or the reason why the promise cannot be fulfilled. ```js findUser().then(function(user){ // user is available }, function(reason){ // user is unavailable, and you are given the reason why }); ``` Chaining -------- The return value of `then` is itself a promise. This second, 'downstream' promise is resolved with the return value of the first promise's fulfillment or rejection handler, or rejected if the handler throws an exception. ```js findUser().then(function (user) { return user.name; }, function (reason) { return 'default name'; }).then(function (userName) { // If `findUser` fulfilled, `userName` will be the user's name, otherwise it // will be `'default name'` }); findUser().then(function (user) { throw new Error('Found user, but still unhappy'); }, function (reason) { throw new Error('`findUser` rejected and we're unhappy'); }).then(function (value) { // never reached }, function (reason) { // if `findUser` fulfilled, `reason` will be 'Found user, but still unhappy'. // If `findUser` rejected, `reason` will be '`findUser` rejected and we're unhappy'. }); ``` If the downstream promise does not specify a rejection handler, rejection reasons will be propagated further downstream. ```js findUser().then(function (user) { throw new PedagogicalException('Upstream error'); }).then(function (value) { // never reached }).then(function (value) { // never reached }, function (reason) { // The `PedgagocialException` is propagated all the way down to here }); ``` Assimilation ------------ Sometimes the value you want to propagate to a downstream promise can only be retrieved asynchronously. This can be achieved by returning a promise in the fulfillment or rejection handler. The downstream promise will then be pending until the returned promise is settled. This is called *assimilation*. ```js findUser().then(function (user) { return findCommentsByAuthor(user); }).then(function (comments) { // The user's comments are now available }); ``` If the assimliated promise rejects, then the downstream promise will also reject. ```js findUser().then(function (user) { return findCommentsByAuthor(user); }).then(function (comments) { // If `findCommentsByAuthor` fulfills, we'll have the value here }, function (reason) { // If `findCommentsByAuthor` rejects, we'll have the reason here }); ``` Simple Example -------------- Synchronous Example ```javascript var result; try { result = findResult(); // success } catch(reason) { // failure } ``` Errback Example ```js findResult(function(result, err){ if (err) { // failure } else { // success } }); ``` Promise Example; ```javascript findResult().then(function(result){ // success }, function(reason){ // failure }); ``` Advanced Example -------------- Synchronous Example ```javascript var author, books; try { author = findAuthor(); books = findBooksByAuthor(author); // success } catch(reason) { // failure } ``` Errback Example ```js function foundBooks(books) { } function failure(reason) { } findAuthor(function(author, err){ if (err) { failure(err); // failure } else { try { findBoooksByAuthor(author, function(books, err) { if (err) { failure(err); } else { try { foundBooks(books); } catch(reason) { failure(reason); } } }); } catch(error) { failure(err); } // success } }); ``` Promise Example; ```javascript findAuthor(). then(findBooksByAuthor). then(function(books){ // found books }).catch(function(reason){ // something went wrong }); ```