Loading a pseudo-class instance from JSON

Question

Is there a better way to do this?

function Person(name)
{
 this._type='Person';
 this.name=name;
 this.hello = function(){
 alert('Hello '+this.name);
 };
}
function object_to_instance(key, value)
{
 if (!value.hasOwnProperty('_type'))
 return value;
 var obj= eval('new '+value._type+'()');
 for (var property in value)
 obj[property]=value[property];
 return obj;
}
var people = [new Person('Harry'), new Person('Sally')];
var people_json = JSON.stringify(people);
var new_people = JSON.parse(people_json, object_to_instance);
new_people[0].hello();

• Bill Barry: instead of eval you could do: var obj = new window[value._type](value.name); (I think you could at least; I don't use classes in js).

Answer 1

Instead of eval, you could create a Person constructor that accepts an object:

function Person(arg)
{
 if (typeof arg === 'string')
 {
 var name = arg;
 this._type='Person';
 this.name=name;
 }
 else if (typeof arg === 'object')
 {
 var value = arg;
 for (var prop in value)
 {
 if (value.hasOwnProperty(prop))
 {
 this[prop] = value[prop];
 }
 }
 }
}
// Move Person.hello() into the prototype
Person.prototype.hello = function()
{
 alert('Hello '+this.name);
};

Then use the _type to choose a constructor based on a predefined set of classes, rather than a (potentially) unsafe eval:

var classes =
{
 Person: Person
 /* other potential classes to deserialize from here as well */
};
function object_to_instance(key, value)
{
 if (!value._type) return value;
 return new classes[value._type](value);
}
var people = [new Person('Harry'), new Person('Sally')];
var people_json = JSON.stringify(people);
var new_people = JSON.parse(people_json, object_to_instance);
new_people[0].hello();

• Basic demo: http://jsfiddle.net/mattball/VfRJn
• Using multiple classes: http://jsfiddle.net/mattball/VfwRv