What is the appropriate way to specify the type of a function parameter to be an express app?

I have a node server running on express and I am looking to add metrics to it during the setup process. Here is a snippet of my code:

const app = express();

installMetrics(app);

While TypeScript can accurately determine the type of app since I have installed @types/express, I am unsure how to properly type the parameter app in my installMetrics function. If I attempt to specify app: Express, I receive an error: "Cannot use namespace 'Express' as a type". I have also considered creating a new type to mimic the usage, but that seems overly complex and not worth the added effort/clutter.

Although I dislike the idea, it appears that using any may be my best option. I have explored other solutions, such as 

declare type TExpress = typeof import("express")
, but it did not provide the correct type (as the import would need to be called to return the accurate type). Am I overlooking something, or is this a issue that I will just have to find a workaround for?

typescriptexpresstypes

Answer №1

By hovering over app, you can observe the inferred type as Express. To ensure it works properly, you need to import it from express:

import express, { Express } from  'express'

const app = express();

installMetrics(app);

function installMetrics(app: Express) {

}

Explore in Playground

Answer №2

When you possess a link to the function that gives back the specific type you desire, TypeScript can help you extract its output and apply it for annotations as required.

type Website = ReturnType<typeof server>;

also

const addAnalytics = (website: Website) => {
  // ...
};

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