btoa() global function

The btoa() method creates a Base64-encoded ASCII string from a binary string (i.e., a string 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.






The binary string to encode.

Return value

An ASCII string containing the Base64 representation of stringToEncode.


InvalidCharacterError DOMException

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



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

Unicode strings

Base64, by design, expects binary data as its input. In terms of JavaScript strings, this means strings in which the code point of 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

For how to work around this limitation when dealing with arbitrary Unicode text, see The "Unicode Problem" section of the Base64 glossary entry.


HTML Standard
# dom-btoa-dev

Browser compatibility

BCD tables only load in the browser

See also