MemoryStore doesn't save ES2015/ES6 Map objects correctly

[AMA3]

When I add a Map object to the session, subsequent requests find an empty object under the same name instead of the original Map. This results in an error.

var express = require('express');
var app = express();
var session = require('express-session')
var MemoryStore = require('memorystore')(session)
app.use(session({
  store: new MemoryStore({ checkPeriod: 86400000 }),
  resave: true,
  saveUninitialized: true,
  secret: 'keyboard cat'
  }));
app.get( '/', ( req, res ) => {
  if ( ! req.session.hasOwnProperty( 'mymap' ) )
    req.session.mymap = new Map( [ [ 'message', 'hello' ] ] );
  console.log( req.session.mymap );
  res.end( req.session.mymap.get('message') )
  } );
app.listen( 8100, () => console.log('Go ahead; I\'m listening.' ) );

First Request (console):

Go ahead; I'm listening.
Map { 'message' => 'hello' }

First Request (browser):

hello

Second Request (console):

TypeError: req.session.mymap.get is not a function
at app.get (.../app.js:15:30)
at Layer.handle [as handle_request] (.../node_modules/express/lib/router/layer.js:95:5)
at next (.../node_modules/express/lib/router/route.js:137:13)
at Route.dispatch (.../node_modules/express/lib/router/route.js:112:3)
at Layer.handle [as handle_request] (.../node_modules/express/lib/router/layer.js:95:5)
at .../node_modules/express/lib/router/index.js:281:22
at Function.process_params (.../node_modules/express/lib/router/index.js:335:12)
at next (.../node_modules/express/lib/router/index.js:275:10)
at Immediate. (.../node_modules/express-session/index.js:489:7)
at runCallback (timers.js:697:11)
{}

Second Request (browser):

TypeError: req.session.mymap.get is not a function
at app.get (.../app.js:15:30)
at Layer.handle [as handle_request] (.../node_modules/express/lib/router/layer.js:95:5)
at next (.../node_modules/express/lib/router/route.js:137:13)
at Route.dispatch (.../node_modules/express/lib/router/route.js:112:3)
at Layer.handle [as handle_request] (.../node_modules/express/lib/router/layer.js:95:5)
at .../node_modules/express/lib/router/index.js:281:22
at Function.process_params (.../node_modules/express/lib/router/index.js:335:12)
at next (.../node_modules/express/lib/router/index.js:275:10)
at Immediate. (.../node_modules/express-session/index.js:489:7)
at runCallback (timers.js:697:11)

I'm using:

"express": "~4.16.0",
"express-session": "^1.15.6",
"memorystore": "^1.6.0",

See also: expressjs/session: es2015 Map object looks problem #302

[roccomuso]

I wasn't able to reproduce this issue.

I'm using express 4.16.3 and node v8.11.1.

PS. also note that the Map obj are not json serializable, and by default memorystore uses JSON.stringify/JSON.parse as serializer.

[AMA3]

Thanks for the response. I'm using Node 10.1.0... dunno if that has anything to do with it. What behavior do you get? In any event I can see that the JSON.stringify/parse limitation could be causing this. Perhaps you should mention it in the readme, either in the introductory paragraphs or under the serializer option explanation?

[roccomuso]

You're right.
Do you mind open a PR?

thanks

[gmcdev]

Agree with OP -- trying to save a Promise to the store also results in an empty object. May not be within the design spec, but should definitely note that the store must be pure JSON.

[HunteRoi]

It would be good to use a work-around to JSONize ES6 Map objects. A simple one is to change the map to an array of arrays containing key-value pair as the constructor of the Map object can take an array of key-value pairs as parameter:

var map = new Map("someKey", { anotherKey: "someValue" });
var jsonizedMap = JSON.stringify([...map]);
var parsedMap = new Map(JSON.parse(jsonizedMap));

Source : https://2ality.com/2015/08/es6-map-json.html