Writing WASM Memory

<!doctype html>
  <title>WASM Test</title>
  <script>
    function fetchAndInstantiateWasm (url, imports) {
      return fetch(url)
      .then(res => {
        if (res.ok)
          return res.arrayBuffer();
        throw new Error(`Unable to fetch Web Assembly file ${url}.`);
      })
      .then(bytes => WebAssembly.compile(bytes))
      .then(module => WebAssembly.instantiate(module, imports || {}))
      .then(instance => instance.exports);
    }

    let mem;
    function writeString (str, offset) {
      const strBuf = new TextEncoder().encode(str);
      const outBuf = new Uint8Array(mem.buffer, offset, strBuf.length);
      for (let i = 0; i < strBuf.length; i++) {
        outBuf[i] = strBuf[i];
      }
    }
    fetchAndInstantiateWasm('https://cdn.rawgit.com/guybedford/wasm-intro/dc93a5f3/5-writing-wasm-memory/program.wasm', {
      env: {
        consoleLog (offset, len) {
          const strBuf = new Uint8Array(mem.buffer, offset, len);
          console.log(new TextDecoder().decode(strBuf));
        }
      }
    })
    .then(m => {
      mem = m.memory;
      writeString("Hello Web Assembly", m.getInStrOffset());
      m.toLowerCase();
    });
  </script>

void consoleLog (char* offset, int len);

char inStr[20];
char outStr[20];

char* getInStrOffset () {
  return &inStr[0];
}

void toLowerCase () {
  for (int i = 0; i < 20; i++) {
    char c = inStr[i];
    if (c > 64 && c < 91) {
      c = c + 32;
    }
    outStr[i] = c;
  }
  consoleLog(&outStr[0], 20);
}