From 9fb14a16fa8f03b2501a9dcabb61bd7ec26a1646 Mon Sep 17 00:00:00 2001
From: Chalarangelo <chalarangelo@gmail.com>
Date: Sat, 18 Sep 2021 21:31:35 +0300
Subject: [PATCH] Add JS callbacks question

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 blog_posts/js-callbacks.md | 33 +++++++++++++++++++++++++++++++++
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+---
+title: What is a callback function?
+type: question
+tags: javascript,function
+authors: chalarangelo
+cover: blog_images/rabbit-call.jpg
+excerpt: JavaScript uses callback functions in various places for different purposes. From event listeners to asynchronous operations, they are an invaluable tool you need to master.
+firstSeen: 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(doublePlusOne); // 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(printDouble); // LOGS: 3, 5, 7
+```