Web/API/WindowOrWorkerGlobalScope/btoa

The WindowOrWorkerGlobalScope.btoa() method creates a Base64-encoded ASCII string from a binary string (i.e., a String object in which each character in the string is treated as a byte of binary data).

You can use this method to encode data which may otherwise cause communication problems, transmit it, then use the atob() method to decode the data again. For example, you can encode control characters such as ASCII values 0 through 31.

Syntax

var encodedData = scope.btoa(stringToEncode);

Parameters

stringToEncode: The binary string to encode.

Return value

An ASCII string containing the Base64 representation of stringToEncode.

Exceptions

InvalidCharacterError: The string contained a character that did not fit in a single byte. See "Unicode strings" below for more detail.

Example

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

Unicode strings

The btoa() function takes a JavaScript string as a parameter. In JavaScript strings are represented using the UTF-16 character encoding: in this encoding, strings are represented as a sequence of 16-bit (2 byte) units. Every ASCII character fits into the first byte of one of these units, but many other characters don't.

Base64, by design, expects binary data as its input. In terms of JavaScript strings, this means strings in which each character occupies only one byte. So if you pass a string into btoa() containing characters that occupy more than one byte, you will get an error, because this is not considered binary data:

const ok = "a";
console.log(ok.codePointAt(0).toString(16)); //   61: occupies < 1 byte

const notOK = "✓"
console.log(notOK.codePointAt(0).toString(16)); // 2713: occupies > 1 byte

console.log(btoa(ok));    // YQ==
console.log(btoa(notOK)); // error

If you need to encode Unicode text as ASCII using btoa(), one option is to convert the string such that each 16-bit unit occupies only one byte. For example:

// convert a Unicode string to a string in which
// each 16-bit unit occupies only one byte
function toBinary(string) {
  const codeUnits = new Uint16Array(string.length);
  for (let i = 0; i < codeUnits.length; i++) {
    codeUnits[i] = string.charCodeAt(i);
  }
  return String.fromCharCode(...new Uint8Array(codeUnits.buffer));
}

// a string that contains characters occupying > 1 byte
const myString = "☸☹☺☻☼☾☿";

const converted = toBinary(myString);
const encoded = btoa(converted);
console.log(encoded);                 // OCY5JjomOyY8Jj4mPyY=

If you do this, of course you'll have to reverse the conversion on the decoded string:

function fromBinary(binary) {
  const bytes = new Uint8Array(binary.length);
  for (let i = 0; i < bytes.length; i++) {
    bytes[i] = binary.charCodeAt(i);
  }
  return String.fromCharCode(...new Uint16Array(bytes.buffer));
}

const decoded = atob(encoded);
const original = fromBinary(decoded);
console.log(original);                // ☸☹☺☻☼☾☿

Polyfill

You can use a polyfill from https://github.com/MaxArt2501/base64-js/blob/master/base64.js for browsers that don't support it.

Specifications

Specification	Status	Comment
HTML Living StandardThe definition of 'WindowOrWorkerGlobalScope.btoa()' in that specification.	Living Standard	Method moved to the `WindowOrWorkerGlobalScope` mixin in the latest spec.
HTML 5.1The definition of 'WindowBase64.btoa()' in that specification.	Recommendation	Snapshot of HTML Living Standard. No change.
HTML5The definition of 'WindowBase64.btoa()' in that specification.	Recommendation	Snapshot of HTML Living Standard. Creation of `WindowBase64` (properties where on the target before it).

Browser compatibility

Update compatibility data on GitHub

	Desktop						Mobile
	Chrome	Edge	Firefox	Internet Explorer	Opera	Safari	Android webview	Chrome for Android	Firefox for Android	Opera for Android	Safari on iOS	Samsung Internet
`btoa`	Chrome Full support 4	Edge Full support 12	Firefox Full support 1 Full support 1 Full support 52 Notes' Notes' `btoa()` now defined on `WindowOrWorkerGlobalScope` mixin.	IE Full support 10	Opera Full support 10.5	Safari Full support 3	WebView Android Full support ≤37	Chrome Android Full support 18	Firefox Android Full support 4 Full support 4 Full support 52 Notes' Notes' `atob()` now defined on `WindowOrWorkerGlobalScope` mixin.	Opera Android Full support 11	Safari iOS Full support 1	Samsung Internet Android Full support 1.0

Legend

Full support: Full support
See implementation notes.': See implementation notes.