How To Encode and Decode Strings with Base64 in JavaScript

Updated on April 15, 2024

Nicholas Cerminara and Anish Singh Walia

How To Encode and Decode Strings with Base64 in JavaScript


In JavaScript, it is possible to use Base64 to encode and decode strings.

In this article, you will be introduced to the btoa and atob JavaScript functions that are available in modern web browsers.

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To follow along with this article, you will need:

Encoding and Decoding Strings with Base64

btoa() and atob() are two Base64 helper functions that are a core part of the HTML specification and available in all modern browsers.

Note: The naming of these functions references old Unix commands for converting binary to ASCII (btoa) and ASCII to binary (atob). However, both the input and output of these functions are Unicode strings.

btoa() takes a string and encodes it to Base64.

Let’s say you have a string, "Hello World!", and wish to encode it to Base64. In your browser’s web developer console, define the string, encode it, and display the encoded string:

// Define the string
var decodedStringBtoA = 'Hello World!';

// Encode the String
var encodedStringBtoA = btoa(decodedStringBtoA);


The output of this code is a string of characters with letters and numbers:


atob() takes a string and decodes it from Base64.

Let’s take the encoded string from earlier, 'SGVsbG8gV29ybGQh', and decode it from Base64. In your browser’s web developer console, define the string, decode it, and display the decoded string:

// Define the string
var encodedStringAtoB = 'SGVsbG8gV29ybGQh';

// Decode the String
var decodedStringAtoB = atob(encodedStringAtoB);


The output of this code reveals the string has been converted back to its original message:

Hello World!

Now, you have two tools for encoding and decoding Base64.

Exploring Common Use Cases for Base64

You can also use Base64 to represent binary data in a way that is compatible with HTML, JavaScript, and CSS. For example, you can embed an image inline in a CSS or JavaScript file using Base64.

It is possible to use Base64 to convert input, like form data or JSON, to a string with a reduced character set that is URL-safe. However, due to how certain servers may interpret plus (+) and forward-slash (/) characters, it is recommended to use encodeURIComponent instead.

Understanding the Limitations of Base64

Base64 is in no way meant to be a secure encryption method.

Base64 is also not a compression method. Encoding a string to Base64 typically results in 33% longer output.

Note: It is important to note that the btoa() and atob() functions only work with strings that contain ASCII characters. If you need to encode or decode strings that contain non-ASCII characters, you should use TextEncoder and TextDecoder classes instead.


In this article, you were introduced to btoa and atob to encode and decode Base64 strings.

Note: Can I Use documents the known issue for handling UTF-8 and provides a polyfill for older browsers.

If you’d like to learn more about JavaScript, check out our JavaScript topic page for exercises and programming projects.

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About the authors
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Nicholas Cerminara


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Sr Technical Writer

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These functions are depreciated.

This comment has been deleted

    doesn’t work for unicode

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