Nest all content into snippets

snippets/articles/s/js-callbacks.md

@@ -0,0 +1,35 @@
+---
+title: What is a callback function?
+shortTitle: Callback functions
+type: question
+language: javascript
+tags: [function]
+author: chalarangelo
+cover: rabbit-call
+excerpt: JavaScript uses callback functions quite a lot. From event listeners to asynchronous code, they're an invaluable tool you need to master.
+dateModified: 2021-10-03T05:00:00-04:00
+---
+
+A callback function is a function passed as an argument to another function, which is then invoked inside the outer function. Callback functions are often executed once an event has occurred or a task has completed.
+
+### Synchronous callbacks
+
+A synchronous callback is a callback function that is executed immediately. The function passed as the first argument to `Array.prototype.map()` is a great example of a synchronous callback:
+
+```js
+const nums = [1, 2, 3];
+const printDoublePlusOne = n => console.log(2 * n + 1);
+
+nums.map(printDoublePlusOne); // LOGS: 3, 5, 7
+```
+
+### Asynchronous callbacks
+
+An asynchronous callback is a callback function that is used to execute code after an asynchronous operation has completed. The function executed inside `Promise.prototype.then()` is a great example of an asynchronous callback:
+
+```js
+const nums = fetch('https://api.nums.org'); // Suppose the response is [1, 2, 3]
+const printDoublePlusOne = n => console.log(2 * n + 1);
+
+nums.then(printDoublePlusOne); // LOGS: 3, 5, 7
+```