How do I make JavaScript Object using a variable String to define the class name?

Question

Here's what I'm trying to do -- this is pseudo code and doesn't work. Does anyone know how to accomplish this for real:

// Define the class
MyClass = Class.extend({});

// Store the class name in a string
var classNameString = 'MyClass';

// Instantiate the object using the class name string
var myObject = new classNameString();

Answer 1

Would it work if you did something like this:

var myObject = window[classNameString];

..?

Answer 2

Here's a more robust solution that will work with namespaced functions:

var stringToFunction = function(str) {
  var arr = str.split(".");

  var fn = (window || this);
  for (var i = 0, len = arr.length; i < len; i++) {
    fn = fn[arr[i]];
  }

  if (typeof fn !== "function") {
    throw new Error("function not found");
  }

  return  fn;
};

Example:

my = {};
my.namespaced = {};
(my.namespaced.MyClass = function() {
  console.log("constructed");
}).prototype = {
  do: function() {
    console.log("doing");
  }
};

var MyClass = stringToFunction("my.namespaced.MyClass");
var instance = new MyClass();
instance.do();

Answer 3

BTW: window is the reference to the global Object in browser JavaScript. Which is also this, and should work even in non-browser environments such as Node.js, Chrome extensions, transpiled code etc.

var obj = new this[classNameString]();

The limitation is that the class being called must be in the global context. If you want to apply the same to a scoped class you need to do:

var obj = (Function('return new ' + classNameString))()

However, there really is no reason to use a string. JavaScript functions are themselves objects, just like strings which are objects also.

Edit

Here is a better way to get the global scope that works in strict mode as well as non-browser JS environments:

var global;
try {
  global = Function('return this')() || (42, eval)('this');
} catch(e) {
  global = window;
}

// and then
var obj = new global[classNameString]

From: How to get the global object in JavaScript?

Answer 4

If MyClass is global, you can access it as a property of window object (assuming your code runs in a browser) using subscript notation.

var myObject = new window["MyClass"]();

Answer 5

If classNameString come from secure source you can use

var classNameString = 'MyClass';
var myObject = eval("new " + classNameString + "()");

This solution works with namespaces and is independent on platform (browser/server).

Answer 6

Here is improved version of Yuriy's method that also handles objects.

var stringToObject = function(str, type) {
    type = type || "object";  // can pass "function"
    var arr = str.split(".");

    var fn = (window || this);
    for (var i = 0, len = arr.length; i < len; i++) {
        fn = fn[arr[i]];
    }
    if (typeof fn !== type) {
        throw new Error(type +" not found: " + str);
    }

    return  fn;
};

Answer 7

function myClass(arg){
}

var str="myClass";
dynamic_class=eval(str);

var instance=new dynamic_class(arg); // OK

Answer 8

On Firefox, there are security rules for extensions, and so for the Javascript console. Don't make the mistake I did to test in the console because none of those solutions work. Whereas when you test from a page it works better :

• the eval solution works well
• Function('return new '... works (object is created) except for the constructor's arguments that are passed as "undefined" I didn't test other solutions.

In French : Sur Firefox il y a des règles de sécurité qui s'appliquent aux extensions, et donc à la console Javascript. Ne faites pas l'erreur que j'ai faite de tester depuis la console car aucune de ces solutions ne marchent. Par contre quand on teste depuis une page cela fonctionne mieux :

• la solution eval fonctionne bien
• Function('return new '... fonctionne (l'objet est créé) sauf que les paramètres du constructeur sont "undefined" Je n'ai pas testé les autres solutions.