Chalarangelo/30-seconds-of-code

---
title: What is the difference between synchronous and asynchronous code in JavaScript?
shortTitle: Synchronous vs asynchronous code
type: question
language: javascript
tags: [function,promises]
cover: pineapple-on-green
excerpt: Understanding the differences between synchronous and asynchronous code is a crucial piece of knowledge for every web developer.
listed: true
dateModified: 2021-11-14
---

Synchronous code runs in sequence. This means that **each operation must wait for the previous one to complete before executing**.

```js
console.log('One');
console.log('Two');
console.log('Three');
// LOGS: 'One', 'Two', 'Three'
```

Asynchronous code runs in parallel. This means that **an operation can occur while another one is still being processed**.

```js
console.log('One');
setTimeout(() => console.log('Two'), 100);
console.log('Three');
// LOGS: 'One', 'Three', 'Two'
```

Asynchronous code execution is often preferable in situations where execution can be **blocked indefinitely**. Some examples of this are network requests, long-running calculations, file system operations etc. Using asynchronous code in the browser ensures the page remains responsive and the **user experience** is mostly unaffected.