Parse JSON String into a Particular Object Prototype in JavaScript

Question

I know how to parse a JSON String and turn it into a JavaScript Object. You can use JSON.parse() in modern browsers (and IE9+).

That's great, but how can I take that JavaScript Object and turn it into a particular JavaScript Object (i.e. with a certain prototype)?

For example, suppose you have:

function Foo()
{
   this.a = 3;
   this.b = 2;
   this.test = function() {return this.a*this.b;};
}
var fooObj = new Foo();
alert(fooObj.test() ); //Prints 6
var fooJSON = JSON.parse({"a":4, "b": 3});
//Something to convert fooJSON into a Foo Object
//....... (this is what I am missing)
alert(fooJSON.test() ); //Prints 12

Again, I am not wondering how to convert a JSON string into a generic JavaScript Object. I want to know how to convert a JSON string into a "Foo" Object. That is, my Object should now have a function 'test' and properties 'a' and 'b'.

UPDATE After doing some research, I thought of this...

Object.cast = function cast(rawObj, constructor)
{
    var obj = new constructor();
    for(var i in rawObj)
        obj[i] = rawObj[i];
    return obj;
}
var fooJSON = Object.cast({"a":4, "b": 3}, Foo);

Will that work?

UPDATE May, 2017: The "modern" way of doing this, is via Object.assign, but this function is not available in IE 11 or older Android browsers.

Answer 1

The current answers contain a lot of hand-rolled or library code. This is not necessary.

1. Use JSON.parse('{"a":1}') to create a plain object.

2. Use one of the standardized functions to set the prototype:

   • Object.assign(new Foo, { a: 1 })
   • Object.setPrototypeOf({ a: 1 }, Foo.prototype)