Typescript: Cannot find namespace

Question

I have the following simple NodeJS code:

const express = require('express');

const server: express.Application = express();

I'm adding Typescript to my project and am new to it so forgive me. With the above code I get the following issues/errors:

For the require:

var require: NodeRequire (id: string) => any (+1 overload)
'require' call may be converted to an import.

For the express.Application usage:

Cannot find namespace 'express'.

If I switch the 'require' to 'import' it fixes the namespace error but is no longer valid Node code so doesn't run (throws a new error about unexpected token for the import).

What's the correct way to write Node code like this with Typescript to avoid these errors?

My tsconfig.json looks like this:

{
  "compilerOptions": {
    "allowJs": true,
    "allowSyntheticDefaultImports": true,
    "alwaysStrict": true,
    "esModuleInterop": true,
    "forceConsistentCasingInFileNames": true,
    "jsx": "preserve",
    "lib": ["dom", "es2017"],
    "module": "esnext",
    "moduleResolution": "node",
    "noEmit": true,
    "noFallthroughCasesInSwitch": true,
    "noImplicitReturns": true,
    "noUnusedLocals": true,
    "noUnusedParameters": true,
    "removeComments": true,
    "resolveJsonModule": true,
    "sourceMap": true,
    "strict": true,
    "target": "esnext",
  },
  "exclude": ["node_modules"],
}

Answer 1

In case you are a React dev that ends up here as I did - if there is JSX syntax in the file, try changing the extension of the file from .ts to .tsx.

Answer 2

After a lot of wasted time, it turns out this was due to the module setting in the tsconfig file which should be:

"module": "commonjs"

This means Typescript will output common js modules instead of ES6 modules, meaning the code will run correctly as NodeJS code. I was therefore able to change require to import, since it compiles.

Answer 3

Maybe you have to use import instead of require:

import * as express from 'express';