Using top-level await with Jest

Don't take my word for it

🚀 Run yarn test to show top-level await working.

Hard-earned lessons

The key things to make sure of are:

• You need jest and ts-jest version 27 or higher.

• These settings need to be in your Jest config:

  "jest": {
    "preset": "ts-jest/presets/default-esm",
    "globals": {
      "ts-jest": {
        "useESM": true
      }
    }
  }

• You need "type": "module" in package.json.

• In tsconfig.json, you need

    "target": "esnext",
    "module": "esnext"

  (The top-level await error says that targeting es2017 or higher would work, but only esnext has worked for me.)

• Jest needs to be run with the --experimental-vm-modules flag. For example my test script looks like this:

  "test": "node --no-warnings --experimental-vm-modules node_modules/jest/bin/jest.js"

  The --no-warnings flag isn't strictly necessary, but otherwise you get yelled at on every test run that --experimental-vm-modules is experimental. 🙄