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I have a constructor function, for example webapp.core.cf1 or Backbone.Router. But I get this constructor function not as a reference, but as a string! I cannot change this requirement, the constr. must be in a string. How can I make a new object from this, for instance classfun("Backbone.Router")?

function classfun (cfstr) 
{ 
  ...
  cf = new window[cfstr]();

If I try it this way, I get the error: ... is not a constructor.

Why this does not work? Is there an alternative way without using eval()?

Thanks a lot in advance

EDIT

Thank you all for your answers!

Thank you, Tronix117, this was the problem!!

Thank you, Benjamin Schulte, for the function!

I have a constructor function, for example webapp.core.cf1 or Backbone.Router. But I get this constructor function not as a reference, but as a string! I cannot change this requirement, the constr. must be in a string. How can I make a new object from this, for instance classfun("Backbone.Router")?

function classfun (cfstr) 
{ 
  ...
  cf = new window[cfstr]();

If I try it this way, I get the error: ... is not a constructor.

Why this does not work? Is there an alternative way without using eval()?

Thanks a lot in advance

EDIT

Thank you all for your answers!

Thank you, Tronix117, this was the problem!!

Thank you, Benjamin Schulte, for the function!

• javascript

Share Improve this question Follow edited Apr 19, 2012 at 13:47 Wolfgang Adamec asked Apr 19, 2012 at 12:46 Wolfgang AdamecWolfgang Adamec 8,5541313 gold badges5151 silver badges7777 bronze badges 3

• Can you give a plete example? Perhaps, on jsfiddle? – Alexander Pavlov Commented Apr 19, 2012 at 12:49
• and also, sample code of what you really want to happen? – Joseph Commented Apr 19, 2012 at 12:50
• How do you call classfun? What type of variable are you passing into it? – Quentin Commented Apr 19, 2012 at 12:51

Add a ment  | 

3 Answers 3

Sorted by: Reset to default 5

If you try in your console:

var A=function(a){this.test=a};
b = new window["A"](3);
console.log(b.test); // return 3

So that means, you are trying to access something which is not in the scope of window, it should rather be something like that: window.something["A"], if you don't know what is something then you should use the function of Benjamin, otherwise use this one, because it's way faster.

You can do it this way (if I understand you correctly):

function getObjectByName(name) {
    var nameParts = name.split('.');
    var nameLength = nameParts.length;
    var scope = window;

    for (var i=0; i<nameLength; ++i) {
        scope = scope[nameParts[i]];
    }

    return scope;
}

var ObjectClass = getObjectByName(cfstr)
new ObjectClass();

Can still be optimized, but should work this way.

ReflectUtil.newInstance = function(strClass) {
    var args = Array.prototype.slice.call(arguments, 1);
    var clsClass = eval(strClass);
    function F() {
        return clsClass.apply(this, args);
    }
    F.prototype = clsClass.prototype;
    return new F();
};

SomeClass = function(arg1, arg2) {
    // ...
}

ReflectUtil.newInstance('SomeClass', 5, 7);

This will allow you to instantiate an object by its name and allow you to pass parameters as necessary.

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