Chaining Promises in Coffeescript - coffeescript

Question

Is there a way to chain Promises together in Coffeescript. For example, consider the following javascript code,
return $.getJSON('/api/post.json')
.then(function(response) {
// do something
})
.then(function(response) {
// do something
})
.then(null, function(err) {
// do something
});
Each of the then's is optional, and the final then needs to be returned by the function.
Currently I am writing this in coffeescript as,
promise = $.getJSON('/api/post.json')
promise = promise.then (response) ->
// do something
promise = promise.then (response) ->
// do something
promise = promise.then null, (err) ->
// do something
return promise
Is there a better way to do this? Thanks.

Answer 1

Ezekiel shows the right way, but it doesn't need the parentheses around the functions. Just do:
$.getJSON '/api/post.json' # As of CoffeeScript 1.7, you don't need the parentheses here either.
.then (response) ->
# do something
response # if you would not return anything, promise would be fulfilled with undefined
.then (response) ->
# do something
undefined # necessary to prevent empty function body
.then null, (err) ->
# handle error
I think it's surprisingly clean.
The one thing that's relatively messy is when you need to add onRejected and onFulfilled handlers at the same time.
Note: Last time I checked, this did not work in CoffeeScript Redux, but this was a few months ago.
Note 2: You need at least one line of actual code (i.e. not just a comment) in each function body for this to work. Typically, you will, so it's not a big issue.

Answer 2

This is my personal favorite way to write promises, with a little bit extra indentation
doSomething = () -> new RSVP.Promise (resolve, reject) ->
if 1 is 1
resolve 'Success'
else
reject 'Error'
doSomething()
.then (res) ->
console.log 'Step 1 Success Handler'
, (err) ->
console.log 'Step 1 Error Handler'
.then (res) ->
console.log 'Step 2 Success Handler'
.then (res) ->
console.log 'Step 3 Success Handler'
, (err) ->
console.log 'Step 3 Error Handler'
Which compiles to:
var doSomething;
doSomething = function() {
return new RSVP.Promise(function(resolve, reject) {
if (1 === 1) {
return resolve('Success');
} else {
return reject('Error');
}
});
};
doSomething().then(function(res) {
return console.log('Step 1 Success Handler');
}, function(err) {
return console.log('Step 1 Error Handler');
}).then(function(res) {
return console.log('Step 2 Success Handler');
}).then(function(res) {
return console.log('Step 3 Success Handler');
}, function(err) {
return console.log('Step 3 Error Handler');
});
There are some instances where this works really well too:
step1Success = (res) -> console.log 'Step 1 Success Handler'
step1Error = (err) -> console.log 'Step 1 Error Handler'
step2Success = (res) -> console.log 'Step 2 Success Handler'
step3Success = (res) -> console.log 'Step 3 Success Handler'
step3Error = (err) -> console.log 'Step 3 Error Handler'
doSomething()
.then(step1Success, step1Error)
.then(step2Success)
.then(step3Success, step3Error)
Tested on coffee-script v1.6.3

Answer 3

This is probably the best you'll do:
$.getJSON('/api/post.json')
.then( (response) ->
# do something
).then( (response) ->
# do something
).then null, (err) ->
# do something
Note the parentheses surrounding the then() arguments. Nothing earth shattering but hopefully this helps.