Base64 Encoding

Fast Base64 encoding and decoding powered by WebAssembly.

Both API and implementation likely to change before the 1.0.0 release.

This library is modeled after the WHATWG TextEncoder and TextDecoder API, providing a Base64Encoder and Base64Decoder class.

The C implementation was chosen based on benchmarks provided by gaspardpetit/base64.


const encoder = await new Base64Encoder().optimize();
encoder.encode(new TextEncoder().encode('foobar'))   // => Zm9vYmFy

const decoder = await new Base64Decoder().optimize();
new TextDecoder().decode(decoder.decode("Zm9vYmFy")) // => foobar

For one-shot usage, you can use the JS implementation without instantiating a WASM instance:

new Base64Encoder().encode(new TextEncoder().encode('foobar'))   // => Zm9vYmFy
new TextDecoder().decode(new Base64Decoder().decode("Zm9vYmFy")) // => foobar

URL-friendly Encoding

This implementation also supports a URL-friendly variant of Base64, where

  • all '+' are mapped to '-'
  • all '/' are mapped to '_'
  • the padding characters '=' are omitted

To use this variant, provide the url setting when creating the encoder.

const encoder = await new Base64Encoder({ url: true }).optimize();

For decoding URL-friendly Base64 no extra steps are required.



Currently only the encoder provides a signification performance improvement over the pure JS implementation.


This module is published on npm under the base64-encoding tag. The package contains the following:

  • The root folder (Browse) exports ES modules in ES2018 syntax. All internal module paths are fully qualified, so they can be imported in Deno or the browser directly.
  • The module folder (Browse) contains a rolled-up version of the above.
  • The cjs folder (Browse) exports CommonJS modules in ES5 syntax.

The root and cjs folder include type declarations and source maps, so that IntelliSense works out of the box in VSCode.

The package.json properly sets the main, module, type and exports keys, so that package.json-based tools will pick the right version.



Currently the C code is licensed under an ancient Apache 1.0 license that comes with some pretty old-school requirements, such as including the following in all promotional materials:

This product includes software developed by the Apache Group for use in the Apache HTTP server project (

It is very likely that ap_base64.c has been shipped under a Apache-2.0 license somewhere. Once I locate it, this requirement will go away.


  • Figure out why decoding is slow
  • License