How to call a class from a javascript string

Question

I am making a nodejs server. I have made a system where I need to dynamically load different classes. The name of the classes are in string. It looks like this:

var classname = "foo"; // this is the name of the class I want to call.

var bar = new classname //classname needs to be foo in this example.

I already tried window[classname] but this wont work because this is nodejs so there is no window to work with.

Thank you for reading :)

score 2 · Answer 1 · answered Sep 19 '17 at 11:14

2

eval("new " + classname) but "beware, eval is evil", etc.

answered Sep 19 '17 at 11:14

Jeremy Thille

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Wow thank you for the link that was very useful. In my case I cant use eval because the classname will come from the url so its not very save. – Rick Grendel Sep 19 '17 at 11:29

score 1 · Accepted Answer · answered Sep 19 '17 at 11:15

1

The better approach is to make use of a JSON object. To achieve that , you can always have a key:value JSON object where the key corresponds to your variable. See the example below. The variable classname is actually a key of the JSON object obj then you can easily reference that to simulate as if you are creating a new class:

var obj = {
   classname : 'foo'
};

var bar = new obj['classname'];

answered Sep 19 '17 at 11:15

Ankit Agarwal

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why `classname` wrap with string in last line? – diEcho Sep 19 '17 at 11:19
because that is not a `variable` and when you are accessing using square brackets without variable name then you need quotes else you can also do `new obj.classname;` – Ankit Agarwal Sep 19 '17 at 11:20
can I use this with es6 classes too? – Rick Grendel Sep 19 '17 at 11:29
Yes you can use it – Ankit Agarwal Sep 19 '17 at 11:30
but that is key and it is constant what if use `new obj[classname]` – diEcho Sep 19 '17 at 11:34
like you can use `new obj[variableHere]` where `variableHere` will have value `classname` then `new obj[variableHere]` will be `new foo` – Ankit Agarwal Sep 19 '17 at 11:36
What is the difference? – Rick Grendel Sep 19 '17 at 11:40
I was just giving you an example. Where you can set the value of `variableHere` dynamically with the `key` name for whose `value` you want to create class – Ankit Agarwal Sep 19 '17 at 11:41
@AKA I used your code but now I get `TypeError: obj.classname is not a constructor` – Rick Grendel Sep 20 '17 at 14:53

How to call a class from a javascript string

2 Answers2