Nodejs, TypeScript, ts-node-dev & top-level await

Question

I have spent an hour now searching for a solution to a new error after needing top-level await. Everything else I have tried so far did not solve the error such as adding "type": "module" to the package.json file.

The message of the error is Cannot use import statement outside a module when starting the service. If I revert the change "module": "ESNext", to "module": "commonjs",, it works fine (except the await keywords have to be removed and somehow refactored to work without await).

In addition, I use ts-node-dev to run the service which can be seen in the package.json file.

• The new package I need is kafkajs.
• node version: v14.9.0
• TypeScript version: 4.0

package.json

{   "name": "",   "version": "1.0.0",   "description": "microservice",   "main": "src/index.ts",   "author": "",   "type": "module",   "license": "MIT",   "scripts": {     "dev": "NODE_ENV=development tsnd --respawn --files src/index.ts",     "prod": "NODE_ENV=production tsnd --respawn --transpile-only --files src/index.ts",     "test": "mocha --exit -r ts-node/register tests/**/*.spec.ts",     "eslint": "eslint src/**/*.ts"   }, 

tsconfig.json

{   "compilerOptions": {     "target": "ES2020",     "module": "ESNext",     "moduleResolution": "node",     "outDir": "dist",     "sourceMap": true,     "esModuleInterop": true,     "allowSyntheticDefaultImports": true,     "experimentalDecorators": true,     "emitDecoratorMetadata": true   },   "ts-node": {     "files": true,     "transpileOnly": true   },   "include": ["src/**/*.ts", "declariations/**.d.ts"],   "exclude": ["node_modules", ".vscode"] } 

Answer 1

TL;DR: Don't use ECMAScript modules with ts-node or ts-node-dev (yet); just refactor the top-level await out

Today I tumbled down the same rabbit hole, starting with the innocuous error:

Top-level 'await' expressions are only allowed when the 'module' option is set to 'esnext' or 'system', and the 'target' option is set to 'es2017' or higher. 

As a result, I felt inclined to edit my tsconfig.json and set module to esnext, which in turn forced me to set moduleResolution to node and finally add type: module to my package.json. What I failed to realize (and IMO they shouldn't suggest this in the error message-- you can just simply refactor the top-level await out), that this switches the module resolution strategy of NodeJS from CommonJS to ESM. This is actually a big deal for many reasons:

• ESM support is still experimental in ts-node
• Mixing CJS and ESM modules may lead to problems
• There are open issues with ESM support in NodeJS itself

I feel that at this time (december 2020) the NodeJS ecosystem is still in transition from the classic CommonJS modules to the new ECMAScript modules. As a result other tech like ts-node or ts-node-dev are transitioning as well and support can be flaky. My advice is to stick with CommonJS until the dust has settled and these things "just work" out of the box.

Answer 2

I know, the question is quite old already, but for people passing by this issue like I did earlier today:

I just managed to get it to work. Not with ts-node-dev directly, but with the combination of nodemon and ts-node.

You need to install nodemon and ts-node and then, assuming your entry file is src/index.ts, you can use this command:

nodemon --ext ts --loader ts-node/esm --experimental-specifier-resolution node src 

(Tested with Node v16.13.0)

This was the key post for me to figure it out: https://github.com/TypeStrong/ts-node/issues/1007