Uint8Array.prototype.toBase64()

Limited availability

This feature is not Baseline because it does not work in some of the most widely-used browsers.

The toBase64() method of Uint8Array instances returns a base64-encoded string based on the data in this Uint8Array object.

This method should be preferred over Window.btoa(), especially if you already have a Uint8Array holding the object, because you don't need to convert it to a string first.

Syntax

js
toBase64()
toBase64(options)

Parameters

options Optional

An object customizing the base64 string format. It can contain the following properties:

alphabet Optional

A string specifying the base64 alphabet to use. It can be one of the following:

"base64" (default)

Encode input with the standard base64 alphabet, which uses + and /.

"base64url"

Encode input with the URL-safe base64 alphabet, which uses - and _.

omitPadding Optional

A boolean specifying whether to omit padding characters (=) at the end of the base64 string. The default is false.

Return value

A base64-encoded string representing the data in the Uint8Array.

Exceptions

TypeError

Thrown in one of the following cases:

  • The options object is not an object or undefined.
  • The options.alphabet is not of the expected values or undefined.

Examples

Encoding binary data

This example uses the default alphabet and omitPadding options to encode data from a Uint8Array into a base64 string.

js
const uint8Array = new Uint8Array([29, 233, 101, 161]);
console.log(uint8Array.toBase64()); // "HelloQ=="

Encoding data without padding

js
const uint8Array = new Uint8Array([29, 233, 101, 161]);
console.log(uint8Array.toBase64({ omitPadding: true })); // "HelloQ"

Encoding data with URL-safe alphabet

This example populates a URLSearchParams object with a base64-encoded string using the URL-safe alphabet.

js
const uint8Array = new Uint8Array([46, 139, 222, 255, 42, 46]);
const base64 = uint8Array.toBase64({ alphabet: "base64url" });
const params = new URLSearchParams();
params.set("data", base64);
console.log(params.toString()); // "data=Love_you"

Stream encoding

This example is adapted from the original proposal, showcasing how to implement streaming in userland. It mimics the TextEncoder API with the stream option.

js
class Base64Encoder {
  #extra;
  #extraLength;
  constructor() {
    this.#extra = new Uint8Array(3);
    this.#extraLength = 0;
  }

  // Partly derived from https://github.com/lucacasonato/base64_streams/blob/main/src/iterator/encoder.ts
  encode(chunk = Uint8Array.of(), options = {}) {
    const stream = options.stream ?? false;

    if (this.#extraLength > 0) {
      const bytesNeeded = 3 - this.#extraLength;
      const bytesAvailable = Math.min(bytesNeeded, chunk.length);
      this.#extra.set(chunk.subarray(0, bytesAvailable), this.#extraLength);
      chunk = chunk.subarray(bytesAvailable);
      this.#extraLength += bytesAvailable;
    }

    if (!stream) {
      // assert: this.#extraLength.length === 0 || this.#extraLength === 3 || chunk.length === 0
      const prefix = this.#extra.subarray(0, this.#extraLength).toBase64();
      this.#extraLength = 0;
      return prefix + chunk.toBase64();
    }

    let extraReturn = "";

    if (this.#extraLength === 3) {
      extraReturn = this.#extra.toBase64();
      this.#extraLength = 0;
    }

    const remainder = chunk.length % 3;
    if (remainder > 0) {
      this.#extra.set(chunk.subarray(chunk.length - remainder));
      this.#extraLength = remainder;
      chunk = chunk.subarray(0, chunk.length - remainder);
    }

    return extraReturn + chunk.toBase64();
  }
}

const encoder = new Base64Encoder();

console.log(
  encoder.encode(Uint8Array.of(72, 101, 108, 108, 111), { stream: true }),
);
// "SGVs"
console.log(
  encoder.encode(Uint8Array.of(32, 87, 111, 114, 108, 100), { stream: true }),
);
// "bG8gV29y"
console.log(encoder.encode());
// "bGQ="

Specifications

Specification
Uint8Array to/from base64
# sec-uint8array.prototype.tobase64

Browser compatibility

BCD tables only load in the browser

See also