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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.


var encodedData = scope.btoa(stringToEncode);


The binary string to encode.

Return value

An ASCII string containing the Base64 representation of stringToEncode.


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


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( 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( Uint16Array(bytes.buffer));

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


You can use a polyfill from for browsers that don't support it.


Specification Status Comment
HTML Living Standard
The definition of 'WindowOrWorkerGlobalScope.btoa()' in that specification.
Living Standard Method moved to the WindowOrWorkerGlobalScope mixin in the latest spec.
HTML 5.1
The definition of 'WindowBase64.btoa()' in that specification.
Recommendation Snapshot of HTML Living Standard. No change.
The 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
ChromeEdgeFirefoxInternet ExplorerOperaSafariAndroid webviewChrome for AndroidFirefox for AndroidOpera for AndroidSafari on iOSSamsung Internet
btoaChrome Full support 4Edge Full support 12Firefox Full support 1
Full support 1
Full support 52
Notes btoa() now defined on WindowOrWorkerGlobalScope mixin.
IE Full support 10Opera Full support 10.5Safari Full support 3WebView Android Full support ≤37Chrome Android Full support 18Firefox Android Full support 4
Full support 4
Full support 52
Notes atob() now defined on WindowOrWorkerGlobalScope mixin.
Opera Android Full support 11Safari iOS Full support 1Samsung Internet Android Full support 1.0


Full support  
Full support
See implementation notes.
See implementation notes.

See also