How to compile TypeScript to JavaScript with top-level await?

Question

I would like to utilize top-level await available since TypeScript 3.8 in a NodeJS application.

So, the following TypeScript code:

import { getDoctorsPage } from "./utils/axios.provider";
const page = await getDoctorsPage("047", "1", "1");
console.log(page);

compiles to this JavaScript code:

import { getDoctorsPage } from "./utils/axios.provider";
const page = await getDoctorsPage("047", "1", "1");
console.log(page);
//# sourceMappingURL=index.js.map

And when I try to run it within WebStorm I get the following error:

(node:95053) ExperimentalWarning: The ESM module loader is experimental.
file:///Users/anatoly/Documents/git/maccabi-parser/dist/index.js:2
const page = await getDoctorsPage("047", "1", "1");
             ^^^^^

SyntaxError: Unexpected reserved word
    at Loader.moduleStrategy (internal/modules/esm/translators.js:81:18)
    at async link (internal/modules/esm/module_job.js:37:21)

Does the latest NodeJS 0.13.* support it?

My tsconfig.json

{
  "compilerOptions": {
    "target": "es2020",
    "module": "esnext",
    "pretty": true,
    "sourceMap": true,
    "outDir": "dist",
    "importHelpers": true,
    "strict": true,
    "moduleResolution": "node",
    "esModuleInterop": true,
    "rootDir": "src",
    "noImplicitAny": false,
    "strictNullChecks": false,
    "noImplicitThis": true,
    "alwaysStrict": true,
    "noUnusedLocals": true,
    "noUnusedParameters": false,
    "noImplicitReturns": true,
    "noFallthroughCasesInSwitch": true,
    "baseUrl": "./",
    "allowSyntheticDefaultImports": true,
    "experimentalDecorators": true,
    "emitDecoratorMetadata": true,
    "resolveJsonModule": true
  },
  "include": [
    "src/**/*"
  ],
  "exclude": [
    "node_modules",
    "dist"
  ]
}

and package.json contains "type": "module"

If you are using this in node, it won't work. node needs the module to be commonjs. — Get Off My Lawn
– Get Off My Lawn, Commented Apr 8, 2020 at 14:54
I guess that it's not just dependent on the version of typescript but also on other things, better check the compatibility or webpack if you're using it and if you need to pass the V8 flag --js-flags="--harmony-top-level-await" — VPaul
– VPaul, Commented Apr 8, 2020 at 14:55
@VPaul you are right. After I've run it as follows: node -r dotenv/config --harmony-top-level-await --es-module-specifier-resolution=node dist/index.js it began to work. I was need to add another flag as well --es-module-specifier-resolution=node otherwise it didn't work with ERR_MODULE_NOT_FOUND. Do you want to write an answer, so I will be able to accept it. It looks that this question could be quite popular — Anatoly
– Anatoly, Commented Apr 8, 2020 at 16:00

VPaul · Accepted Answer · 2020-04-08 16:21:11Z

2

As mentioned in the comments that this feature is not just dependent on the version of TS but more things like version of WebPack (if using WebPack) and existence of the V8 flag --js-flags="--harmony-top-level-await" during execution of the program or in configs.

Executing this code with this flag enabled can be done as mentioned in the comments:

node -r dotenv/config --harmony-top-level-await --es-module-specifier-resolution=node dist/index.js

answered Apr 8, 2020 at 16:21

VPaul

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