feat(content): add async await js article

src/content/resources/javascript/promises/async-await.md

@@ -0,0 +1,138 @@
+---
+authors:
+  - "ddivad#1337"
+created_at: "2019/10/01"
+title: Async Await
+recommended_reading:
+  - javascript/promises/intro
+---
+
+## Async Await
+
+Async / Await is another way to handle the calling of asynchronous code and is built on top of Promises. It is a nice and clean way to handle asynchronous code, without the need to `.then()` functions to the promise.
+
+It is made up of 2 keywords as the name suggests: `async` and `await`. These need to be used together and can for this method to work.
+
+## Async
+
+The `async` keyword is used to show that the function is going to return a promise. Any return values from the function will be converted to a promise automatically, if they are not already.
+
+```js
+async function foo() {
+  return "foo";
+}
+```
+
+This basic example will return a promise with the value of `foo`.
+
+## Await
+
+The `await` keyword is used to wait until a promise has executed and fetches the result. In order to use the `await` keyword, you **need** to be inside an `async` function. 
+
+```js
+async function demo() {
+  let data = await foo();
+  console.log(foo); // "foo"
+}
+```
+
+This code will call the `foo()` function from above and will wait on that line until the promise returned from the `async` function has returned.
+
+Another example to demonstrate this is if we extended the `foo()` function from above to add a 5 second delay.
+
+```js
+async function foo() {
+  let promise = new Promise((resolve, reject) => {
+    setTimeout(() => resolve("foo"), 5000)
+  });
+
+  let data = await promise; // Execution waits here until promise resolves
+  console.log(data); // "foo"
+}
+```
+
+## Error Handling
+
+As with Promises, if the Promise returns with an error, it will affect the `await` call that triggered it. When using Promises without Async / Await, the `.catch()` syntax is used, and this also exists with Aysnc / Await using the `try...catch` syntax. This can be demonstrated by making our `foo()` function throw an error.
+
+```js
+async function foo() {
+  throw new Error("An Error Occurred");
+}
+```
+
+This would be handled using Aysnc / Await like so:
+
+```js
+async function getData() {
+  try {
+    let data = await foo();
+    console.log(data);
+  } catch (err) {
+    console.error(err); // err: An Error Occurred
+  }
+}
+```
+
+## Beginner Mistakes
+
+As mentioned in the [intro](./intro.md), Promises are the highest source of confusion for beginners. Async / Await adds another layer on top of Promises, and comes with its own pitfalls.
+
+`await` will **only** work if the function you try to add `await` to is `async`. Using `await` in top level code will not work (yet).
+
+```js
+let data = await foo(); // syntax error
+```
+
+To get around this issue, you can use an declare asynchronous function anonymously, if needed:
+
+```js
+(async () => {
+  let data = await foo();
+  console.log(data); // "foo"
+})();
+```
+
+Top-Level-Await is something that *may* get added to Javascript in the future, but for now wrapper functions like above are needed for this functionality.
+
+## Real World Example
+
+Here is a real function that gets character information from the Rick and Morty API.
+You can try it in your browser if you want to test it out.
+
+```js
+async function getCharacters() {
+  const response = await fetch(`https://rickandmortyapi.com/api/character`);
+  const data = await response.json();
+  return data.results;
+}
+
+(async () => {
+  try {
+    let characters = await getCharacters();
+    console.log(characters) // (20) [{...}, {...}, {...}]
+  } catch (err) {
+    console.error(err);
+  }
+})();
+```
+
+In this example, we are querying real data from the Rick & Morty API. This API has a lot of information regarding Rick & Morty, but here, we are trying to retrieve all the characters. 
+- The first thing we do is use `fetch` to retrieve the data from the API. `fetch` is a built in function in web browsers to do http requests, and returns a Promise by default. We `await` the result of this.
+- When we get the result, we need to get the `JSON` value of the data. To do this, the `.json()` method from fetch is used. We then return the results.
+- As `await` can only be used in an `async` function, we use the method from above to make this work. As `getCharacters()` returns a Promise due to it being `async`, we `await` the result. We surround this in a `try...catch` in case fetch returns an error. Then, if no error is returned, `characters` contains the information we want, and is logged to the console. If `getCharacters()` returns an error, that error is also logged.
+
+## Comparision with default Promises:
+
+Below is the same code implemented without using Async / Await as a comparision. Here, you can see the differences between the two methods, and how Aysnc / Await makes the code appear in a *more synchronous* pattern, and can be clearer to follow.
+```js
+function getCharacters() {
+  return fetch(`https://rickandmortyapi.com/api/character`)
+    .then(response => response.json())
+    .then(response => response.results)
+}
+
+getCharacters().then(characters => {
+  console.log(characters) // (20) [{...}, {...}, {...}]
+}).catch(err => console.error(err))
+```