Saving the file was as easy as creating a new Buffer with the Uint8Array object :

// chunk is the Uint8Array object
fs.appendFile(path, Buffer.from(chunk), function (err) {
    if (err) {
      fut.throw(err);
    } else {
      fut.return(chunk.length);
    }
});

You cannot read a file by pathname through a browser. You need to have the user interact with the file system and choose a file before you can read the content.

const readFile = e => {
  const file = e.target.files[0]
  let reader = new FileReader();

  reader.onload = function(e) {
      let arrayBuffer = new Uint8Array(reader.result);
      console.log(arrayBuffer);
  }

  reader.readAsArrayBuffer(file);
}

document.querySelector("#fileItem").onchange=readFile

<input id="fileItem" type="file">