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

Browser global object is window and whenever you define global variables with var or functions with function, you are adding them in window. Thus you can get your "class" definition there:

var args = [];
var className = 'MyClass';
var obj = new window[className](args);

But this won't work for ES6 class declarations

Classes declared using ES6 keyword class are per-standard treated differently.

A class declared with class MyClass { } defines a global class that does not become a property of window global object. In other words the following applies

class MyClass {};
typeof window.MyClass === 'undefined';

So, how to do the same with ES6 classes? Object access notation is required because is what is needed to parse the string name, but parent object to search in is no longer available.

One way is to create your own context object, declare there your class and search for it there. In code:

// this variable actually goes in `window`
var classes = {};
// declare class inside
classes.MyClass = class {
   // the class code
};

var args = [];
var className = 'MyClass';
var obj = new classes[className](args); // dynamic for "new classes.MyClass(args)"

Answer 7

function myClass(arg){
}

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

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

Edit: inline example

function Person(name){
    this.name=name;
}
var person1=new (eval("Person"))("joe");

Answer 8

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 9

This works:

import { Class1 } from './profiles/class1.js'
import { Class2 } from './profiles/class2.js'

let profiles = {
  Class1,
  Class2
}

let profileAsString = 'Class1'
new profiles[profileAsString]()

Answer 10

In my case I was preloading data from my server and attempting to load the preloaded data using classes (in particular, I am using vuex-orm). Instead of anything super fancy, I opted for a list of models that I expect, and mapped them to classes that I had already imported at the top of the file. Observe:

import Video from '@models/Video'
import Purchase from '@models/Purchase'

let modelClassMap = {
    'Video': Video,
    'Purchase': Purchase,
}

Object.entries(preload.models).forEach(entry => {
    const [modelName, modelData] = entry
    if(modelClassMap[modelName]){
        modelClassMap[modelName].insertOrUpdate({data: modelData})
    }
})

Explicit, secure, simple. Nice!

Answer 11

myOption = {.......}

new (eval( varNameClass ))({ param0:'xxx', myThis: this, option: myOption })

Answer 12

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.