How to declare a function parameter type as an express app?-CodePudding

I have a node server which uses express, and I need to install metrics on it during setup. So I have something like:

const app = express();

installMetrics(app);

TypeScript is able to determine the type of app fine, as I've installed @types/express. But in declaring my function installMetrics, I don't know how to type the parameter app. If I try app: Express, I get an error: "Cannot use namespace 'Express' as a type". I could create a new type to mock the usage, but that's cumbersome, does not at all seem worth the effort/clutter.

As much as I hate to, it seems like my best option is to just use any. I've tried other solutions that I've found here such as declare type TExpress = typeof import("express") and it didn't get the correct type (because the import would need to be called to return the correct type). Am I missing something, or is this just a hole I'll have to patch over?

CodePudding user response：

If you hover over app you can see the inferred type which is Express. If you import it from express it will work:

import express, { Express } from  'express'

const app = express();

installMetrics(app);

function installMetrics(app: Express) {

}

Playground Link

CodePudding user response：

Since you have a reference to the function that returns the type you're interested in, you can have TypeScript extract its return value and use that to annotate as needed.

type App = ReturnType<typeof express>;

and

const installMetrics = (app: App) => {
  // ...
};