[SOLVED] How to create an express Application with import instead of require

Issue

This is what works with require. We instead want it to use import.

import { Request, Response, Application } from 'express';

// TODO Figure out how NOT to use require here.
const express = require('express');
var app: Application = express();

app.get('/', function (req: Request, res: Response) {
  res.send('Hello World')
});

app.listen(3000);

What we have tried

Our tsconfig.json has "esModuleInterop": true.

Attempt # 1

import express from 'express';

That gives this error:

“node_modules/@types/express/index”‘ has no default export.ts

Attempt # 2

import * as express from 'express';
var app = express();

That gives a different error:

Cannot invoke an expression whose type lacks a call signature. Type ‘typeof e’ has no compatible call signatures.ts(2349)
index.ts(1, 1): Type originates at this import. A namespace-style import cannot be called or constructed, and will cause a failure at runtime. Consider using a default import or import require here instead.

Solution

TypeScript has special import syntax to deal with modules that export functions or some other custom objects as a whole (not as default export):

import { Request, Response, Application } from 'express';
import express = require('express');

var app: Application = express();

app.get('/', function (req: Request, res: Response) {
  res.send('Hello World')
});

app.listen(3000);

---

Alternatively you can use TypeScript compiler options to alter imported module so that they have default export:

// tsconfig.json
{
  "compilerOptions": {
    "allowSyntheticDefaultImports": true,     /* Allow default imports from modules with no default export. This does not affect code emit, just typechecking. */
    "esModuleInterop": true,                  /* Enables emit interoperability between CommonJS and ES Modules via creation of namespace objects for all imports. Implies 'allowSyntheticDefaultImports'. */
  }
}

and import using this default import:

import express from 'express' 
var app = express();

Answered By – Kamil Szot

Answer Checked By – Cary Denson (BugsFixing Admin)