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# Promise
`Promise` is a class available in the Opal stdlib for helping structure asynchronous code.
It can be required inside any Opal applicaton:
```ruby
require 'promise'
```
_Please also take a look at the Asynchronous code guide - we are in the process of modernizing the Promises, along with supporting async/await_
## Usage
This example shows how to use a `HTTP` request from `opal-jquery` from a callback style, into a promise style handler.
```ruby
def get_json(url)
promise = Promise.new
HTTP.get(url) do |response|
if response.ok?
promise.resolve response.json
else
promise.reject response
end
end
promise
end
get_json('/users/1.json').then do |json|
puts "Got data: #{json}"
end.fail do |res|
alert "It didn't work :( #{res}"
end
```
A promise can only be resolved or rejected once.
### Chaining Promises
Promises become useful when chained together. The previous example could be extended to get another object from the result of the first request.
```ruby
get_json('/users/1.json').then do |json|
get_json("/posts/#{json[:post_id]}.json")
end.then do |post|
puts "got post: #{post}"
end
```
### Composing Promises
`Promise.when` can be used to wait for more than 1 promise to resolve (or reject). Lets assume we wanted to get 2 different users:
```ruby
first = get_json '/users/1.json'
second = get_json '/users/2.json'
Promise.when(first, second).then do |user1, user2|
puts "got users: #{user1}, #{user2}"
end.fail do
alert "Something bad happened"
end
```
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