How to run and debug flow a script in node using babel

Currently Node.js support may ES6 features but full support for modules (import and export) is still missing. Similarly Node.js can’t directly execute Flow type-annotated scripts. Standard solution is transpilation to node accepted form of Javascript. In case one wants to avoid this intermediate step, babel-node is a solution.

babel-node allows transparent in-memory transpilation and interpretation of Javascript using node. It allows both launching a script file and REPL mode. (Authors warn it’s not for production use.)

To run flow annotated ES6 module install required npm packages (it may be installed globally as well):

npm install babel-cli babel-preset-env babel-preset-flow

Run script using:

node_modules/.bin/babel-node --presets env,flow script.js

Debugging in babel-node

babel-node works with --inspect-brk option eventhough it is not mentioned on its web or in output of help.

1. Run

   node_modules/.bin/babel-node --inspect-brk --presets env,flow script.js
   

2. Use any node debugging UI. In case of Visual Studio Code:

   1. Open debug panel (Menu > View > Debug)

   2. Make sure attaching configuration is present.
      Debug configurations (Menu > Debug > Open Configurations) should contain a configuration like

      {
          "type": "node",
          "request": "attach",
          "name": "Attach by Process ID",
          "processId": "${command:PickProcess}"
      }
      

   3. Select the attaching configuration, start debugging (Menu > Debug > Start Debugging), select babel-node process.