JavaScript factory with optional arguments

Question

The problem is that I need to create a new instance of the passed class

Is there a way to rewrite this function so that it can accept any number of arguments?

function createInstance(ofClass, arg1, arg2, arg3, ..., argN){ return new ofClass(arg1, arg2, arg3, ..., argN); } 

This function should create an instance of a past class. Example:

 var SomeClass = function(arg1, arg2, arg3){ this.someAttr = arg3; ..... } SomeClass.prototype.method = function(){} var instance = createInstance(SomeClass, 'arg1', 'arg2', 'arg3'); 

So that must be true.

 instance instanceof SomeClass == true 

Right now, I have limited N to 25, hoping that more arguments are rarely used.

Answer 1

Other answers are on the right track, but none of them mention that you need to know that arguments not Array . This is a special structure that behaves like an Array .

So, before using it as an Array , you can convert it to one of the following:

 function createInstance(cls) { // This will use the slice function of the Array type, effectively converting // the arguments structure to an Array and throwing away the first argument, // which is cls. var args = Array.prototype.slice.call(arguments, 1); return cls.apply(this, args); } 

Sorry, I just copied the code with constructor , etc. and didn’t think about what he would actually do. I updated it now to what you want. You will find that it invokes the constructor without new , so you will not get the same behavior. However, John Resig (author of jQuery) wrote on this very topic .

So, based on an article by John Resig, you have two ways to solve this problem. A more complex solution will be the most transparent to the user, but it depends on which solution you choose.

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Here is the “perfect” solution if you only intend to support browsers that have the Object.create function (now this is a fairly large percentage compared to three years earlier):

 function createInstance(cls) { var obj = Object.create(cls.prototype); var args = Array.prototype.slice.call(arguments, 1); cls.apply(obj, args); return obj; } 

The resulting objects from new cls(x) and createInstance(cls, x) must be identical.

Answer 2

There is always an array