Use of instanceof in TypeScript

Question

Consider this basic node+ts code:

import * as express from "express";

function doStuff(app: express.Application) {
  if (!(app instanceof express.Application)) throw new TypeError();
  // etc...
}

VSCode shows me this error for ...instanceof express.Application...:

[ts] Property 'Application' does not exist on type 'typeof e'. [2339]

What am I doing wrong?

Titian Cernicova-Dragomir · Accepted Answer · 2019-01-20 07:51:31Z

4

instanceof is a Javascript operator. The right hand side operand must be a runtime value (a function or a constructor) for it to work.

express.Application is defined as an interface. Interfaces only exist at compile time to aid in type checking. This means at runtime, there is no express.Application value to be the operand in the instanceof operation, and so typescript issues an error.

answered Jan 20, 2019 at 7:51

Titian Cernicova-Dragomir

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2 Comments

lonix Over a year ago

Thanks that makes sense. I could use duck typing to achieve the same result, though that's messy... is there an alternative?

Titian Cernicova-Dragomir Over a year ago

@Ionix As the type is defined you can only check if specific properties exist

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