TypeScript: can only be default-imported using the 'esModuleInterop' flag

Question

I've the following index.ts:

import { User } from "./datatypes"
import express from 'express';

 console.log("hello world")

This is my package.json:

{
  "name": "simple_app",
  "version": "1.0.0",
  "description": "",
  "main": "index.js",
  "scripts": {
    "start": "tsc index.ts && node index.js"
  },
  "keywords": [],
  "author": "",
  "license": "ISC",
  "dependencies": {
    "dotenv": "^16.0.0",
    "express": "^4.17.3"
  },
  "devDependencies": {
    "@types/express": "^4.17.13",
    "@types/node": "^17.0.23",
    "ts-node": "^10.7.0",
    "typescript": "^4.6.3"
  }
}

And this is my tsconfig.json:

{
    "compilerOptions": {
      "module": "commonjs",
      "esModuleInterop": true,
      "resolveJsonModule": true,
      "target": "es6",
      "moduleResolution": "node",
      "sourceMap": true,
      "outDir": "dist"
    },
    "lib": ["es2015"]
  }

When I write npm run start, I obtain:

 Module '"/mnt/c/Users/raffa/Desktop/simple_app/node_modules/@types/express/index"' can only be default-imported using the 'esModuleInterop' flag

2 import express from 'express';
         ~~~~~~~

  node_modules/@types/express/index.d.ts:133:1
    133 export = e;
        ~~~~~~~~~~~
    This module is declared with using 'export =', and can only be used with a default import when using the 'esModuleInterop' flag.

How can I solve this problem?

Floppy

in package .json if I substitute "start": "tsc index.ts && node index.js" with "start": "tsc -w && node index.js", it works, but in the terminal, I can't see anything (I can't see console.log("hello world") – Floppy

Answer 1

If you call tsc from the command line with a specific file path, it will ignore your tsconfig.json

Typescript docs (https://www.typescriptlang.org/docs/handbook/tsconfig-json.html):

When input files are specified on the command line, tsconfig.json files are ignored.

This means that the esModuleInterop part of tsconfig.json will have no effect if you call tsc index.ts.

It's possible to work around it by using import * as express from 'express';, but there may be other issues arising from ignored TS configs.

One problem with tsc -w && node index.js (from one of the comments) is that the -w will make it wait instead of terminating by itself, which means you won't get to the part after the "&&".
So, you won't get to the node index.js part.
(But at least this time, tsc doesn't get one specific file, therefore the config is not ignored)

Also, in your config, the outDir is set to dist, so the compiled file will go there. So, you should use the following instead:
tsc && node ./dist/index.js

Scarron

Answer 2

I had this error and just restarted VS Code

but the error also appeared all of a sudden in the first place (i.e. I already had a working project that I ran successfully a couple of times)

Resuscitator

Answer 3

I got this error when I left out some inner quotes in my tsconfig.json:

"include": ["src/**/*.ts, src/**/*.tsx"]  # bad
"include": ["src/**/*.ts", "src/**/*.tsx"]  # good

By omitting the quotes, I got a glob that didn't match anything, and therefore didn't import node_modules/@types/react/index.d.ts, so I couldn't do import React from 'react'. Fixing the quotes fixed it.

Initiatory