Global

Methods

addAsync()

addAsync() immediately invokes the Promise constructor. The actual implementation of that function resides in the callback that is passed to that constructor (line A). That callback is provided with two functions:
  • resolve is used for delivering a result (in case of success).
  • reject is used for delivering an error (in case of failure).
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call_1_getData()

Calls getData()

To actually consume the value returned when the promise fulfills, since it is returning a promise, we could use a .then() block.

If the promise gets rejected, it will jump to the catch( ) method and this time we will see a different message on the console.
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call_2_doSomething()

Calls doSomething()

Making asynchronous programming easier with async and await. 
 Usage with two possible outcomes, depending on which one resolves first,
 getData() or do something():

 roma: ~/html/cwdc/3-javascript/promises$ node promise.js

 getData resolved with some data (after 5.868s)
 some data (after 10.869s)
 doSomething resolved with value 3

 or

 some data (after 6.588s)
 doSomething resolved with value 3
 getData resolved with some data (after 10.494s)

 rejected

 took too long (11.309s)
 doSomething resolved with value 3
 getData resolved with some data (after 4.477s)
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call_addAsync()

Calls addAsync.

Promises are similar to the event pattern: There is an object (a Promise), where we register callbacks:
  • Method .then() registers callbacks that handle results.
  • Method .catch() registers callbacks that handle errors.
   sum = 5
   Error: Must provide two parameters
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(async) doSomething() → {Promise.<Number>}

An async function can wait for a promise to be resolved.
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Returns:
returned value 3 wrapped as a promise.
Type
Promise.<Number>

getData() → {Promise.<String>}

When you create a Promise, you pass in a callback function as an argument.

Inside the function, you pass in two arguments: resolve and reject.
When the Promise’s should be considered completed, run the resolve() method.

The Promise.resolve() method "resolves" a given value to a Promise:
  • If the value is a promise, that promise is returned;
  • if the value is a thenable, Promise.resolve() will call the then() method with two callbacks it prepared;
  • otherwise the returned promise will be fulfilled with the value.
The function passed to new Promise is called the executor.
When new Promise is created, the executor runs automatically.
It contains the producing code which should eventually produce the result.
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Returns:
a promise for returning a string after at most 12 seconds.
Type
Promise.<String>

(async) showAvatar() → {Object}

Retrieves a user avatar from github, and draws it for 3 seconds.

The user is read from a user.json file in the current directory, using the fetch API:

   fetch('http://example.com/movies.json')
       .then(response => response.json())
       .then(data => console.log(data));
 
   user.json file contents:

   {
     "name": "jimblin",
     "isAdmin": true
   }
All the processing is asynchronous.
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Returns:
a github user. 
   {
     login: "Jimblin",
     id: 75284734,
     node_id: "MDQ6VXNlcjc1Mjg0NzM0",
     avatar_url: "https://avatars.githubusercontent.com/u/75284734?v=4",
     gravatar_id: "",
     …
   }
Type
Object