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snippets/async-javascript-cheatsheet.md

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+---
+title: Asynchronous JavaScript Cheat Sheet
+type: cheatsheet
+tags: [javascript,function,promise]
+author: chalarangelo
+cover: green-plant
+excerpt: Learn everything you need to know about promises and asynchronous JavaScript with this handy cheatsheet.
+dateModified: 2021-06-12T19:30:41+03:00
+---
+
+### 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
+  }
+}
+```