Serialization of a polymorphic array in node.js - node.js

Question

I am looking for a method for serialization of a Javascript object, that contains several other objects of different classes, with function arguments.
Here is a simple test-case:
// Paper class:
function Paper(name) {
this.name = name;
}
Paper.prototype = {
string: function() { return "Paper:"+this.name; }
};
// Book class:
function Book(name) {
this.name = name;
}
Book.prototype = {
string: function() { return "Book:"+this.name; }
};
// Library class:
function Library(name) {
this.items = [];
}
Library.prototype = {
add: function(item) { this.items.push(item); },
string: function () {
var titles = this.items.map(function(item) { return item.string(); });
return titles.join(",");
},
};
///// Define a library:
var lib = new Library();
lib.add(new Paper("MyPaper"));
lib.add(new Book("MyBook"));
assert(lib.string() == "Paper:MyPaper,Book:MyBook");
///// Serialize, de-serialize and compare:
// var libString = serialize(lib);
// var newLib = deserialize(libString);
// assert(newLib.string() == "Paper:MyPaper,Book:MyBook");
NOTE: The main usage of de/serialization (at least in my case) is for moving complex objects to distant computers. For example, I want to build a big Library on my computer, then serialize it, put on a file, send the file to another computer, deserialize it there, and have the exact same Library.

Answer 1

You need to extend JSON semantics. If I were you, I would to something like this:
var protos = {}, //hash of prototypes
base = { //base prototype
toJSON: function () {
var props = {}; //properties to be serialized
for (var prop in this) { //this can be custimized, like taking `attrs` hash or something
if (this.hasOwnProperty(prop)) props[prop] = this[prop];
}
props.$proto = this.$proto; //need to copy this manually since it's not an `own propery`
return props;
}
};
function Paper(name) {
this.name = name;
}
protos.paper = Paper.prototype = Object.create(base);
Paper.prototype.$proto = 'paper';
Paper.prototype.toString = function() {
return 'Paper: ' + this.name;
}
function Book(name) {
this.name = name;
}
protos.book = Book.prototype = Object.create(base);
Book.prototype.$proto = 'book';
Book.prototype.toString = function() {
return 'Book: ' + this.name;
}
function Library(name) {
this.items = [];
}
Library.prototype = {
add: function(item) { this.items.push(item); },
toString: function () {
var titles = this.items.map(function(item) {
return item.toString();
});
return titles.join(',');
},
toJSON: function () {
return this.items.map(function(item) { return item.toJSON()});
}
};
Library.fromJSON = function (json) {
return json.map(function(item) {
var object = Object.create(protos[item.$proto]);
for (var prop in item) object[prop] = item[prop];
return object;
});
};
//test
var lib = new Library();
lib.add(new Paper('MyPaper'));
lib.add(new Book('MyBook'));
console.log(lib.toString());
var json = JSON.stringify(lib.toJSON());
console.log(Library.fromJSON(JSON.parse(json)).toString());
Here is a fiddle: http://jsfiddle.net/cSTT5/

Answer 2

I want to build a big Library on my computer, then serialize it, put on a file, send the file to another computer, deserialize it there, and have the exact same Library.
I've made an npm module named esserializer to solve this problem: save JavaScript class instance values recursively during serialization, in plain JSON format, together with its class name information. Then, during the deserialization stage, esserializer can recursively deserialize object instance, with all types/functions information retained, using the same class definition.
For your scenario, the code is pretty simple. It works if the serialized string is taken to another computer, as long as the same class definition exists on that machine:
const ESSerializer = require('esserializer');
// Paper class:
function Paper(name) {
this.name = name;
}
Paper.prototype = {
string: function() { return "Paper:"+this.name; }
};
Paper.prototype.constructor = Paper; // We need to redefine the constructor of Paper's prototype.
// Book class:
function Book(name) {
this.name = name;
}
Book.prototype = {
string: function() { return "Book:"+this.name; }
};
Book.prototype.constructor = Book;
// Library class:
function Library(name) {
this.items = [];
}
Library.prototype = {
add: function(item) { this.items.push(item); },
string: function () {
var titles = this.items.map(function(item) { return item.string(); });
return titles.join(",");
},
};
Library.prototype.constructor = Library;
///// Define a library:
var lib = new Library();
lib.add(new Paper("MyPaper"));
lib.add(new Book("MyBook"));
// assert(lib.string() == "Paper:MyPaper,Book:MyBook");
var serializedString = ESSerializer.serialize(lib);
// Later, on another machine, deserializedObj is a perfect copy of "lib":
var deserializedObj = ESSerializer.deserialize(serializedString, [Library, Book, Paper]);
console.log(deserializedObj.string()); // Paper:MyPaper,Book:MyBook