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 = "โ" 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 = "โธโนโบโปโผโพโฟ"; 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); // โธโนโบโปโผโพโฟ
Polyfill
You can use a polifill from https://github.com/MaxArt2501/base64-js/blob/master/base64.js for browsers that don't support it.
Specifications
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. |
HTML5 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
Desktop | Mobile | |||||||||||
---|---|---|---|---|---|---|---|---|---|---|---|---|
btoa | Chrome Full support 4 | Edge Full support 12 | Firefox
Full support
1
| IE Full support 10 | Opera Full support 10.5 | Safari Full support 3 | WebView Android Full support โค37 | Chrome Android Full support 18 | Firefox Android
Full support
4
| 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.