content/snippets/js/s/async-cheatsheet.md
---
title: Asynchronous JavaScript Cheat Sheet
type: cheatsheet
language: javascript
tags: [promises,function,cheatsheet]
cover: green-plant
excerpt: Learn everything you need to know about promises and asynchronous JavaScript with this handy cheatsheet.
listed: true
dateModified: 2021-06-12
---
## Promise basics
- **Promises** start in a **pending state**, neither fulfilled or rejected.
- When the operation is completed, a promise will become **fulfilled with a value**.
- If the operation fails, a promise will get **rejected with an error**.
## Creating promises
- The function passed to the `Promise` constructor will execute synchronously.
- Use `resolve()` or `reject()` to create promises from values.
- `Promise.resolve(val)` will fulfill the promise with `val`.
- `Promise.reject(err)` will reject the promise with `err`.
- If you put a fulfilled promise into a fulfilled promise, they will collapse into one.
```js
// Resolving with a value, rejecting with an error
new Promise((resolve, reject) => {
performOperation((err, val) => {
if (err) reject(err);
else resolve(val);
});
});
// Resolving without value, no need for reject
const delay = (ms) => new Promise(resolve => setTimeout(resolve, ms));
```
## Handling promises
- `Promise.prototype.then()` accepts two optional arguments (`onFulfilled`, `onRejected`).
- `Promise.prototype.then()` will call `onFulfilled` once the promise is fulfilled.
- `Promise.prototype.then()` will call `onRejected` if the promise is rejected.
- `Promise.prototype.then()` passes errors through if `onRejected` in undefined.
- `Promise.prototype.catch()` accepts one argument (`onRejected`).
- `Promise.prototype.catch()` behaves like `Promise.prototype.then()` when `onFulfilled` is omitted.
- `Promise.prototype.catch()` passes fulfilled values through.
- `Promise.prototype.finally()` accepts one argument (`onFinally`).
- `Promise.prototype.finally()` calls `onFinally` with no arguments once any outcome is available.
- `Promise.prototype.finally()` passes through input promise.
```js
promisedOperation()
.then(
val => value + 1, // Called once the promise is fulfilled
err => { // Called if the promise is rejected
if (err === someKnownErr) return defaultVal;
else throw err;
}
)
.catch(
err => console.log(err); // Called if the promise is rejected
)
.finally(
() => console.log('Done'); // Called once any outcome is available
);
```
- All three of the above methods will not be executed at least until the next tick, even for promises that already have an outcome.
## Combining promises
- `Promise.all()` turns an array of promises into a promise of an array.
- If any promise is rejected, the error will pass through.
- `Promise.race()` passes through the first settled promise.
```js
Promise
.all([ p1, p2, p3 ])
.then(([ v1, v2, v3 ]) => {
// Values always correspond to the order of promises,
// not the order they resolved in (i.e. v1 corresponds to p1)
});
Promise
.race([ p1, p2, p3 ])
.then(val => {
// val will take the value of the first resolved promise
});
```
## async/await
- Calling an `async` function always results in a promise.
- `(async () => value)()` will resolve to `value`.
- `(async () => throw err)()` will reject with an error.
- `await` waits for a promise to be fulfilled and returns its value.
- `await` can only be used in `async` functions.
- `await` also accepts non-promise values.
- `await` always waits at least until the next tick before resolving, even when waiting already fulfilled promises or non-promise values.
```js
async () => {
try {
let val = await promisedValue();
// Do stuff here
} catch (err) {
// Handle error
}
}
```