messenger-to-markdown.js

// http://bgrins.github.io/devtools-snippets/#console-save
var saveToFile = function(data, filename) {
  if (!data) {
    console.error('Console.save: No data');
    return;
  }

  if (!filename) {
    filename = 'console.json';
  }

  if (typeof data === "object") {
    data = JSON.stringify(data, undefined, 4);
  }

  var blob = new Blob([data], {type: 'text/json'}),
      e    = document.createEvent('MouseEvents'),
      a    = document.createElement('a');

  a.download = filename;
  a.href = window.URL.createObjectURL(blob);
  a.dataset.downloadurl =  ['text/json', a.download, a.href].join(':');
  e.initMouseEvent('click', true, false, window, 0, 0, 0, 0, 0, false, false, false, false, 0, null);
  a.dispatchEvent(e);
};

var nodes = [].slice.apply(document.querySelectorAll("[aria-label=Messages] [data-hover=tooltip]"))
var name = document.querySelector("[aria-label=Messages] img[alt]").getAttribute("alt")

var content = nodes.map(e => ({
  name: e.getAttribute("data-tooltip-position") === "left" ? name : "Jamie Wong",
  time: e.getAttribute("data-tooltip-content"),
  content: e.textContent
})).filter(v => v.content.length > 0)

saveToFile(content.map(function(msg) {
  var content = "**" + msg.name + "**" + " *" + msg.time + "*\n" + msg.content;
  return content.split("\n").map(function(l) { return "> " + l; }).join("\n");
}).join("\n>\n"), "chatlog.txt");