How can I store a Map object in a Chrome App?

Question

Using Chrome API chrome.storage.local, I can save and successfully retreive an array but I cannot retreive a Map object.

var folders = new Map()
//... populate Map
chrome.storage.local.set( { "myFolders": folders } )

chrome.storage.local.get( "myFolders", function ( saved ) 
{
  console.assert( typeof saved.myFolders.size === 'number', "not a Map!" )
} )

I'm forced to convert Map in Array before storing. Could I store Map objects directly ?

Answer 1

No, you can't store, or pass with Messaging, objects that are not JSON-serializable (DOM nodes being another frequent example).

And a Map is not:

> JSON.stringify(new Map().set("a", "b"))
"{}"

So, you can only store what JSON can encode. That means that you'll have to do your own serialization/deserialization on top of storage access.

Edit: as Simon's answer shows, Chrome performs more elaborate serialization than JSON (preserving RegExp and Date), but the principle still stands: non-primitive objects need custom serialization.

Answer 2

Well, check the documentation:

Primitive values such as numbers will serialize as expected. Values with a typeof "object" and "function" will typically serialize to {}, with the exception of Array (serializes as expected), Date, and Regex (serialize using their String representation).

So you cannot do that directly without converting the data.