Check if object(s) in TypeScript file implement specific interface

Question

I'm trying to create a process that looks at a bunch of *.ts files in a folder and determines which ones contain a class that implements a specific interface. While I'd like to be able to include multiple classes in one file, I'm fine if I can only do one.

At present, I'm simply loading the files, calling the constructor and then checking for the existence of a few properties, but some of these files may contain enums which are throwing exceptions.

I know interfaces don't have a runtime equivalent so I can't do a type check against them, but is there another option to check if what's in a file is a class / enum?

EDIT: The scenario is that I have an interface that defines an Express.js route. I then have a single class that looks at all the files in a folder and will load each one. For each file, I want to see "if it is of type IRouteDefinition then dynamically load its route & handler"

Can you provide a little bit more detail, e.g. why you want to check the type of a class? If you only want to do some static analysis it would suggest using the TS language service. — Sebastian Sebald
– Sebastian Sebald, Commented Aug 18, 2016 at 21:19
Maybe I didn't describe it right... the language service is not what I want to use... this needs to happen at runtime.The scenario is that I have an interface that defines an Express.js route. I then have a single class that looks at all the files in a folder and will load each one. For each file, I want to see "if it is of type IRouteDefinition then dynamically load it's route & handler" — Andrew Connell
– Andrew Connell, Commented Aug 19, 2016 at 10:20
Yeah, but as you already said: interfaces do not exist at runtime. So I guess you have to use the language service at runtime. Or a completely other approach would be to "tag" such route definitions via decorators. — Sebastian Sebald
– Sebastian Sebald, Commented Aug 19, 2016 at 11:18
Oh... that's interesting... decorating the class like we do in ng2. Any pointers on where to read up on creating custom decorators? — Andrew Connell
– Andrew Connell, Commented Aug 19, 2016 at 20:50

basarat · Accepted Answer · 2016-08-18 22:31:31Z

2

I know interfaces don't have a runtime equivalent so I can't do a type check against them, but is there an other option to check if what's in a file is a class / enum

This sort of thing is what the language service is for. However the learning curve is a bit steep (but totally worth it!)

Docs

Some compiler docs : https://basarat.gitbooks.io/typescript/content/docs/compiler/overview.html
A video on the language service : https://www.youtube.com/watch?v=WOuNb2MGR4o
An IDE just to make it easier for you to play with the lang service : http://alm.tools/ e.g. checkout the documentation view https://basarat.gitbooks.io/alm/content/features/documentation-view.html which really took me a weekend to write 🌹.

answered Aug 18, 2016 at 22:31

basarat

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Sebastian Sebald · Accepted Answer · 2016-08-19 21:09:35Z

1

As mentioned in one of my comments, you could use decorators to do this. The TypeScript Docs got you covered on how to create a custom decorator: https://www.typescriptlang.org/docs/handbook/decorators.html

Here is an example implementation to get you started: http://jsbin.com/voyemu/3/edit?js,console

answered Aug 19, 2016 at 21:09

Sebastian Sebald

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