YUI Library Examples: JSON Utility: Rebuilding class instances from JSON data

The CaveMan class

For this example, we'll use a class CaveMan, with a property discovered that holds a Date instance, and a method getName.

Add `freeze` and `thaw` static methods

We'll add the methods responsible for serializing and reconstituting instances to the CaveMan class as static methods.

Reference the methods in replacer and reviver functions

We'll create a demo.FreezeThaw namespace to hold our moving parts. In it, we'll add a method to pass to JSON.stringify that calls our custom serializer, and another method to pass to JSON.parse that detects the serialized structure and calls our thawing method.

The data to be serialized

We'll create a CaveMan instance and nest it in another object structure to illustrate how the thawing process still operates normally for all other data.

Thawing from the inside out and the `Date` instance

The reviver function passed to JSON.parse is applied to all key:value pairs in the raw parsed object from the deepest keys to the highest level. In our case, this means that the name and discovered properties will be passed through the reviver, and then the object containing those keys will be passed through.

We'll take advantage of this and apply YAHOO.lang.JSON.stringToDate to all string values, which will reconstitute Date instances when they match the iso8601 UTC format that JSON serializes Dates to by default.

Now when the reviver function is evaluating the object it determines to be a CaveMan, the discovered property is correctly containing a Date instance.

Choose your serialization

You'll note there are two freeze and thaw operations going on in this example. One for our CaveMan class and one for Date instances. Their respective serialization and recreation techniques are different. You are free to decide the serialized format of your objects. Choose whatever makes sense for your application.

Show and Tell

Now we add the event handlers to the example buttons to call JSON.stringify and parse with our example.cryo and example.revive methods, respectively.

Full Code Listing

YAHOO.util.Event.onDOMReady(function () {

var Event = YAHOO.util.Event,
    Dom   = YAHOO.util.Dom,
    JSON  = YAHOO.lang.JSON,
    Demo;
    
Demo = YAHOO.namespace('demo').FreezeThaw = {
    data       : null,
    jsonString : null,

cryo : function (k,o) {
        return (o instanceof CaveMan) ? CaveMan.freeze(o) : o;
    },
    revive : function (k,v) {
        // Turn anything that looks like a UTC date string into a Date instance
        if (typeof v === 'string') {
            return JSON.stringToDate(v);;
        }
        // Check for cavemen by the _class key
        if (v instanceof Object && v._class == 'CaveMan') {
            return CaveMan.thaw(v);
        }
        // default to returning the value unaltered
        return v;
    }
};

function CaveMan(name,discovered) {
    this.name       = name;
    this.discovered = discovered;
};
CaveMan.prototype.getName = function () {
    return this.name + ", the cave man";
}

// Static methods to convert to and from a basic object structure
CaveMan.thaw = function (o) {
    return new CaveMan(o.n, o.d);
};
// Convert to a basic object structure including a class identifier
CaveMan.freeze = function (cm) {
    return {
        _class : 'CaveMan',
        n : cm.name,
        d : cm.discovered // remains a Date for standard JSON serialization
    };
};

Demo.data    = {
    count : 1,
    type  : 'Hominid',
    specimen : [
        new CaveMan('Ed',new Date(1946,6,6))
    ]
};

Event.on('demo_freeze','click',function (e) {
    Demo.jsonString = JSON.stringify(Demo.data, Demo.cryo);

Dom.get('demo_frozen').value = Demo.jsonString;
    Dom.get('demo_thaw').disabled = false;
});

Event.on('demo_thaw','click',function (e) {
    var parsedData = JSON.parse(Demo.jsonString, Demo.revive);
        cm = parsedData.specimen[0];

Dom.get('demo_thawed').innerHTML =
 "Specimen count: " + parsedData.count + ""+
 "Specimen type: " + parsedData.type + ""+
 "Instanceof CaveMan: " + (cm instanceof CaveMan) + ""+
 "Name: " + cm.getName() + ""+
 "Discovered: " + cm.discovered + "";
});

});

YUI Logger Output:

Note: Logging and debugging is currently turned off for this example.

Reload with logging
and debugging enabled.

JSON Utility Examples:

More JSON Utility Resources:

All YUI 2.x users should review the YUI 2.8.2 security bulletin, which discusses a vulnerability present in YUI 2.4.0-2.8.1.