Attempting to utilize a namespace-style import for calling or constructing purposes will result in a runtime failure

Using TypeScript 2.7.2 and VSCode version 1.21 with @types/express, I encountered an issue where in certain cases VSCode would report errors like:

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

Interestingly, on other machines with similar setups and identical tsconfig.json files, the code runs without any issues. What could be the reason for this inconsistency?

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

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

Any insights on why this discrepancy occurs?

Thanks in advance,

John.

typescriptexpresstypescript-typings

Answer №1

If you are encountering the error specifically with esModuleInterop: true, it could be possible that VSCode is not recognizing your tsconfig.json file or there might be another configuration file overriding it with esModuleInterop: true.

To permanently resolve this issue, ensure that the compiler option esModuleInterop is set to true and change your import statement to import express instead of import * as express.

The Issue at Hand :

Under the ES6 specification, a "namespace object" is defined by statements like 

import * as namespaceObject from 'a-module'
. According to the spec, the typeof this object is supposed to be an object, which should not be callable.

Prior to this, you were using import * as express because express is a CommonJS module exported using module.exports. However, this practice is against the ES6 standard, causing TypeScript to issue warnings about it.

The Resolution Approach :

By enabling `esModuleInterop` to true, TypeScript will handle your import calls by determining if the module is in ES6 or CommonJS format. In case of a CommonJS module being used alongside import default from 'module', TypeScript can accurately retrieve the CommonJS module.

According to the TypeScript release note:

Note: The new behavior is added under a flag to avoid unintended disruptions to existing code bases. We strongly recommend applying it to both new and current projects. For established projects, namespace imports (import * as express from "express"; express();) should be converted to default imports (import express from "express"; express();).

Answer №2

When I encountered this issue, my 

"esModuleInterop": true
setting was already activated in the tsconfig.json file. To resolve the issue, I had to change the following code:

import * as assert from "assert";

to:

import assert from "assert";

Answer №3

Can we try this instead:

const express = require('express');

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