blake2b.js

async function setup() {
  // In node, this will use the polyfill and read the file from disk using `fs.readFile('blake2b.wasm', ...)`
  // But in the browser, it will make an HTTP fetch request to `GET blake2b.wasm`!
  let res = await fetch('blake2b.wasm')
  let buf = await res.arrayBuffer()
  let mod = await WebAssembly.instantiate(buf))
  // ...
}

node-fetch.js

// A tiny fetch polyfill for node that allows loading external resources in isomorphic libraries.
// Run node code with `-r ./node-fetch` to enable polyfill without needing to change source code.

let { readFile } = require('fs')

global.fetch = fetch

let P = (fn, ...args) => new Promise((resolve, reject) =>
  fn(...args, (err, val) => err ? reject(err) : resolve(val)))

async function fetch (path) {
  let data = await P(readFile, path)

  return {
    code: 200,
    arrayBuffer
  }

  async function arrayBuffer () {
    return new Uint8Array(data)
  }
}