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content(resources): update/tweak JS promises resources
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@@ -1,53 +1,60 @@
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---
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authors:
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- "ddivad#0001"
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- "veksen#1060"
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created_at: "2019/10/01"
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title: Async Await
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updated_at: 2020/05/20
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title: Async/Await
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recommended_reading:
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- javascript/promises/intro
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---
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## Async Await
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## Async/Await
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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.
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Async/Await is another way to handle the calling of asynchronous code and is built on top of Promises. It makes it possible to write asynchronous code that feels synchronous.
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**Its usage is equivalent to resolving a promise with `.then()`**.
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It is made up of 2 keywords as the name suggests: `async` and `await`. These need to be used together for this method to work.
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## Async
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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.
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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.
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```js
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async function foo() {
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return "foo";
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async function getWeather() {
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return "sunny";
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}
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getPromise();
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// Promise<"sunny">
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```
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This basic example will return a promise with the value of `foo`.
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This basic example will return a promise with the value of `getWeather`.
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## Await
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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.
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```js
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async function demo() {
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let data = await foo();
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console.log(foo); // "foo"
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async function checkWeather() {
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const weather = await getWeather();
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console.log(weather); // "sunny"
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}
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```
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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.
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This code will call the `getWeather()` function from above and will wait on that line until the promise returned from the `async` function has returned.
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Another example to demonstrate this is if we extended the `foo()` function from above to add a 5 second delay.
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Another example to demonstrate this is if we extended the `getWeather()` function from above to add a 5 second delay.
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```js
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async function foo() {
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let promise = new Promise((resolve, reject) => {
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setTimeout(() => resolve("foo"), 5000);
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async function getWeather() {
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const promise = new Promise((resolve, reject) => {
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setTimeout(() => resolve("sunny"), 5000);
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});
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let data = await promise; // Execution waits here until promise resolves
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console.log(data); // "foo"
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const weather = await promise; // Execution waits here until promise resolves
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console.log(weather); // "sunny"
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}
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```
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@@ -81,15 +88,15 @@ As mentioned in the [intro](./intro.md), Promises are the highest source of conf
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`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).
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```js
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let data = await foo(); // syntax error
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let weather = await getWeather(); // syntax error
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```
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To get around this issue, you can use an declare asynchronous function anonymously, if needed:
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```js
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(async () => {
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let data = await foo();
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console.log(data); // "foo"
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let weather = await getWeather();
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console.log(weather); // "sunny"
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})();
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```
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@@ -107,6 +114,14 @@ async function getCharacters() {
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return data.results;
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}
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// or, mixed
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async function getCharacters() {
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const data = await fetch(
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`https://rickandmortyapi.com/api/character`
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).then((res) => res.json());
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return data.results;
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}
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(async () => {
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try {
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let characters = await getCharacters();
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@@ -123,15 +138,28 @@ In this example, we are querying real data from the Rick & Morty API. This API h
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- 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.
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- 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.
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## Comparision with default Promises:
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### Let's also look at an example from Discord.js:
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```js
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client.on("message", (msg) => {
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if (msg.author.bot) return;
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if (msg.content === "ping") {
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const message = await msg.channel.send("pong");
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message.react("⚠️");
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}
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})
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```
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## Comparision with default Promises
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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.
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```js
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function getCharacters() {
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return fetch(`https://rickandmortyapi.com/api/character`)
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.then((response) => response.json())
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.then((response) => response.results);
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.then((res) => res.json())
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.then((res) => res.results);
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}
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getCharacters()
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@@ -1,8 +1,9 @@
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---
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authors:
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- "Xetera#0001"
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- "veksen#1060"
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created_at: 2019/07/26
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updated_at: 2019/07/26
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updated_at: 2020/05/20
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title: Introduction to Promises
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recommended_reading:
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- javascript/callbacks/intro
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@@ -85,6 +86,14 @@ getWeather("Los Angeles").then((weather) => {
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});
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```
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Outlining this one again, because it's very common, is to attempt to use the value of a promise, but not resolving it:
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```js
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// Incorrect code, don't copy!
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const weather = getWeather("Los Angeles");
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console.log(weather); // Promise {<pending>}
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```
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## Real World Example
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Here is a real function that gets character information from the Rick and Morty API.
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