Instantiate JS Object that take array of arguments from String name

Question

I have a JS class that takes in several arguments:

var MyClassName = function(arg1, arg2, arg3){ // Do stuff };

I have a String that represents the class's name, and an array of arguments.

var class = "MyClassName"
var args = ["1", "2", "3"]

Using the String of the class name and the array of arguments, how can I instantiate the class? Prototype Bind/apply seems to have some relevance, but I can't quite figure out how this all works together. I've also tried using window["MyClassName"], but that throws an undefined error. Any suggestions?

possible duplicate of What techniques can be used to define a class in JavaScript, and what are their trade-offs? — RafaelKr
– RafaelKr, Commented Jun 1, 2015 at 23:19
To clarify, I'm trying instantiate a new instance of a class based on a string of the class's name. The class takes several arguments, but I only want to pass it an array or arguments. — ph34r
– ph34r, Commented Jun 1, 2015 at 23:26
Can you explain the reason for doing this? That might help someone offer a better solution. — Yogi
– Yogi, Commented Jun 1, 2015 at 23:51

Giuseppe Pes · Accepted Answer · 2015-06-02 07:47:31Z

You can use eval:

var a = eval("new " + className + " ( " + arguments.toString() + ")");

Example:

JSFIDDLE: https://jsfiddle.net/s40ave7r/

function A(a1,a2,a3) {
    this.a1 = a1;
    this.a2 = a2;
    this.a3 = a3;
}

var className = "A";
var arguments = [1,2,3];

var a = eval("new " + className + " ( " + arguments.toString() + ")");

console.log(a); // A {a1:1, a2:2, a3:3}

Note:

Using eval in general is a bad idea. If you can try to avoid it. These are some problems related to eval:

Improper use of eval opens up your code for injection attacks
Debugging can be more challenging (no line numbers, etc.)
eval'd code executes more slowly (no opportunity to compile/cache eval'd code)

Solution 2

Other solution is to attach the class definition to an object. For example, when you define a class in the global scope this is attached by default to the window object in the browser, or to the global object in node. So you can do :

Browser

function MyClass() {}
new window["MyClass"](arg1, arg2..);

Node

function MyClass() {}
new window["MyClass"](arg1, arg2..);

RafaelKr · Accepted Answer · 2015-06-02 14:41:21Z

You can define your 'class' as

window.MyClassName = function( arg1, arg2, ... ) {
    this.arg1 = arg1;
    //...
};

then you can call it with

var instance = new window['MyClassName']( arg1, arg2, ... )

or with

var instance = new window.MyClassName( arg1, arg2, ... );

or better don't use window, use an own object.

var classes = {
    MyClassName: function( arg1, arg2, ... ) {
        this.arg1 = arg1;
        //...
    }
};

var instance = new classes.MyClassName( arg1 );

Edit: You said window["MyClassName"] didn't work, but if you define a 'class' in the global scope, then you can call it like this. Can you please add the error you meant?

function testClass( arg1 ) {
    this.foo = arg1;
}

var instance = new window['testClass']( 'bar' );
console.log( instance.foo ); // The output will be bar

This code is working, but probably you forgot the 'new'. If you try this without the 'new', then it throws this error at Chrome:

Uncaught TypeError: Cannot read property 'foo' of undefined

Community · Accepted Answer · 2017-05-23 11:43:40Z

0

Thank you all for the wonderful suggestions. I combined part of the answers here in addition to this SO answer:

var className = new window['MyClassName'] var instantiatedClass = className.bind.apply(className, [null, arg1, arg2, ...]);

edited May 23, 2017 at 11:43

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answered Jun 3, 2015 at 0:16

ph34r

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