Window: atob() method

{{APIRef("HTML DOM")}}

The atob() method of the {{domxref("Window")}} interface decodes a string of data which has been encoded using {{glossary("Base64")}} encoding. You can use the {{domxref("Window.btoa()")}} method to encode and transmit data which may otherwise cause communication problems, then transmit it and use the atob() method to decode the data again. For example, you can encode, transmit, and decode control characters such as {{Glossary("ASCII")}} values 0 through 31.

Also consider using the {{jsxref("Uint8Array.fromBase64()")}} method, which creates a Uint8Array object from a base64-encoded string. It results in a byte array, which is easier to work with than a string containing raw bytes.

Syntax

atob(encodedData)

Parameters

• encodedData
  • : A base64-encoded string, using the alphabet produced by {{domxref("Window.btoa()")}}.

Return value

A binary string containing raw bytes decoded from encodedData. Strings in JavaScript are encoded as {{glossary("UTF-16")}}, so this means each character must have a code point less than 256, representing one byte of data.

Exceptions

• InvalidCharacterError {{domxref("DOMException")}}
  • : Thrown if encodedData is not valid base64.

Examples

const encodedData = window.btoa("Hello, world"); // encode a string
const decodedData = window.atob(encodedData); // decode the string

For more examples, see the {{domxref("Window.btoa()")}} method.

Specifications

{{Specifications}}

Browser compatibility

{{Compat}}

See also

• A polyfill of atob is available in core-js
• data URLs
• {{domxref("WorkerGlobalScope.atob()")}}: the same method, but in worker scopes.
• {{domxref("Window.btoa()")}}
• {{jsxref("Uint8Array.fromBase64()")}}