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# Example: Rebuilding Class Instances from JSON Data

This example illustrates one method of serializing and recreating class instances by using the `replacer` and `reviver` parameters to `JSON.stringify` and `JSON.parse` respectively.

    (stringify results here)

### 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`.

    YUI().use("node", "json", function(Y) {

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

    ...

### Add `freeze` and `thaw` static methods

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

    // Static method to convert to a basic structure with a class identifier
    CaveMan.freeze = function (cm) {
        return {
            _class : 'CaveMan',
            n : cm.name,
            d : cm.discovered // remains a Date for standard JSON serialization
        };
    };

    // Static method to reconstitute a CaveMan from the basic structure
    CaveMan.thaw = function (o) {
        return new CaveMan(o.n, o.d);
    };

### Reference the methods in replacer and reviver functions

We'll create an `example` 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.

    var example = {
        cryo : function (k,o) {
            return (o instanceof CaveMan) ? CaveMan.freeze(o) : o;
        },

        revive : function (k,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;
        }
    };

### 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.

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

### 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 by watching for UTC formatted date strings (the default JSON serialization for Dates) and reviving them into proper `Date` instances before the containing object gets its turn in the reviver.

    var example = {
        dateRE : /^(\d{4})-(\d{2})-(\d{2})T(\d{2}):(\d{2}):(\d{2})(?:\.\d+)?Z$/,

        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
            var match = Y.Lang.isString(v) ? v.match(example.dateRE) : null,
                d;

            if (match) {
                d = new Date();
                d.setUTCFullYear(match[1], (match[2] - 1), match[3]);
                d.setUTCHours(match[4], match[5], match[6]);
                return d;
            }
            // 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;
        }
    };

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 very different. You are free to decide the serialized format of your objects. Choose whatever makes sense for your application.

Note: There is no explicit `Date` serialization method listed inline because `JSON` natively supports `Date` serialization. However, it is outside the scope of the parser's duty to create Date instances, so it's up to you to recreate them in the `parse` phase. Feel free to use the method included here.

### 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.

    Y.one('#demo_freeze').on('click',function (e) {
        example.jsonString = Y.JSON.stringify(example.data, example.cryo);

        Y.one('#demo_frozen').set('innerHTML', example.jsonString);
        Y.one('#demo_thaw').set('disabled',false);
    });

    Y.one('#demo_thaw').on('click',function (e) {
        var x  = Y.JSON.parse(example.jsonString, example.revive);
            cm = x.specimen[0];

        Y.one('#demo_thawed').set('innerHTML',
            "<p>Specimen count: " + x.count + "</p>"+
            "<p>Specimen type: " + x.type + "</p>"+
            "<p>Instanceof CaveMan: " + (cm instanceof CaveMan) + "</p>"+
            "<p>Name: " + cm.getName() + "</p>"+
            "<p>Discovered: " + cm.discovered + "</p>");
    });

    }); // end of YUI(..).use(.., function (Y) {

### Full Code Listing

    <style scoped>
    #demo pre {
        background: #fff;
        border: 1px solid #ccc;
        margin: 1em;
        padding: 1em;
        white-space: pre-wrap; /* css-3 */
        word-wrap: break-word; /* Internet Explorer 5.5+ */
    }
    </style>

    <div id="demo">
        <input type="button" id="demo_freeze" value="Freeze">
        <input type="button" id="demo_thaw" disabled="disabled" value="Thaw">
        <pre id="demo_frozen">(stringify results here)</pre>
        <div id="demo_thawed"></div>
    </div>

    <script type="text/javascript">
    YUI().use("node", "json", function(Y) {

    var example = {
        data       : null,
        jsonString : null,

        dateRE : /^(\d{4})-(\d{2})-(\d{2})T(\d{2}):(\d{2}):(\d{2})(?:\.\d+)?Z$/,

        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
            var match = Y.Lang.isString(v) ? v.match(example.dateRE) : null,
                d;

            if (match) {
                d = new Date();
                d.setUTCFullYear(match[1], (match[2] - 1), match[3]);
                d.setUTCHours(match[4], match[5], match[6]);
                return d;
            }
            // 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
        };
    };

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

    Y.one('#demo_freeze').on('click',function (e) {
        // Format the string with 4 space indentation
        example.jsonString = Y.JSON.stringify(example.data, example.cryo, 4);

        Y.one('#demo_frozen').set('text', example.jsonString);
        Y.one('#demo_thaw').set('disabled',false);
    });

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

        Y.one('#demo_thawed').set('innerHTML',
            "<p>Specimen count: " + parsedData.count + "</p>"+
            "<p>Specimen type: " + parsedData.type + "</p>"+
            "<p>Instanceof CaveMan: " + (cm instanceof CaveMan) + "</p>"+
            "<p>Name: " + cm.getName() + "</p>"+
            "<p>Discovered: " + cm.discovered + "</p>");
    });

    // Expose the example objects for inspection
    example.CaveMan = CaveMan;
    YUI.example = example;

    });
    </script>