A namespace-style import cannot be called or constructed, and will cause a failure at runtime

Question

Strange stuff happening here, with TypeScript 2.7.2, in VSCode version 1.21 with @types/express and the code that follows, in some cases VSCode throws errors stating that "A namespace-style import cannot be called or constructed, and will cause a failure at runtime.". However on other machines with similar setups and similar tsconfig.json files the code just works.. What is happening here:

import { Bank } from './Bank';
import * as Express from 'express';  <== errors here..

let app: Express.Express;
this.app = Express();                <== and here

Why is this happening?

TIA,

John.

Answer 1

The error should only happen with esModuleInterop: true. Maybe VSCode is ignoring your tsconfig.json or another one with esModuleInterop: true is used.

For a definitive fix set the compiler option esModuleInterop to true and use import express instead of import * as express.

The problem :

The ES6 specification defines the notion of "namespace object". A namespace object is namespaceObject in this statement : import * as namespaceObject from 'a-module'. The typeof this object is object, you are not supposed to be able to call it.

You were using import * as express until now because express is a CommonJS module, for Node.js, it is exported using module.exports. However it is illegal in the ES6 spec, and TypeScript now warns you.

The solution :

Setting esModuleInterop to true will make TypeScript wrap your import calls to check if the module is an ES6 module or a CommonJS. If it's a CommonJS module and you are using import default from 'module' TypeScript will find out and return the correct CommonJS module.

From the TypeScript release note :

Note: The new behavior is added under a flag to avoid unwarranted breaks to existing code bases. We highly recommend applying it both to new and existing projects. For existing projects, namespace imports (import * as express from "express"; express();) will need to be converted to default imports (import express from "express"; express();).