Avoid double-transformation of a file when multiple ts-node hooks are registered

[sixmen]

Maintainer edit: If you would like to submit a pull request implementing this feature, I have described a straightforward implementation here: #409 (comment)

If I require 'ts-node/register' twice, it does not run well. For examples,

my.ts:

export default class My {
  msg: string;
  constructor() {
    this.msg = 'hello';
  }
}

index.js:

require('ts-node/register');
My = require('./my').default;
console.log(new My().msg);

If I run with node index.js, it works. But if I run with ts-node index.js, it fails.

In the real world, I run app with ts-node.
And config module, which I use, also register ts-node ( https://github.com/lorenwest/node-config/blob/master/lib/config.js#L872 ).

[blakeembrey]

There's no great way to do this except for throwing an error and making it impossible for your code to execute. If I silently allow one and not the other, especially in this case, it's pretty likely the compiler options or configuration options are different and that'll result in an inconsistent runtime. I'd recommend you use a different module (there should be no reason for a config module to alter your runtime, it should do transpilation separately) or make an issue on that module.

I'll keep this open to remind me to throw an error if you try to double register in the future. This will not solve your problem though.

[electroma]

I face similar issue with "config" module. It registers ts-node explicitly https://github.com/lorenwest/node-config/blob/02c98b7f5947ece93357652b69d9cb3da6decfd3/lib/config.js#L878

@blakeembrey, after i read your comment it is clear that fix should be done on "config" side.
Can you please suggest the best way to make sure that ts-node is already registered?

[electroma]

@sixmen node-config/node-config#458 should fix your issue

[ZSkycat]

I face similar issue with webpack-cli.

ts-node --project ./tool/tsconfig.json  ./node_modules/webpack/bin/webpack.js --config ./tool/config.webpack.prod.ts

I modify the config file path with ts-node. But webpack-cli re-register ts-node.
When CWD equals ./, it loads the config file ./tsconfig.json. overwrite my config file ./tool/tsconfig.json.

If there is a way to detect whether it is registered, the caller can solve this problem.

[blakeembrey]

@ZSkycat Do you have a suggestion for how this could be detected? My only thought is storing a global that can be accessed somehow by all modules, but I have no consensus on where this should be stored.

FWIW, you should look at using environment variables in that case too - it's why they're provided.

[ZSkycat]

@blakeembrey
I don't know how to use it (environment variables). and it makes the command longer.
jonaskello/tsconfig-paths#48

So, I use it in a strange way.
cd tool && webpack --config config.webpack.dev.ts

[blakeembrey]

@ZSkycat It looks like two issues:

1. You might misunderstand cross-env - you need to remove the && since it would be executing ts-node without the environment variable (says run x and, when successful, run y - x && y)
2. tsconfig-paths appears to be parsing as JSON, which means you can't have any of TypeScript's enhancements to tsconfig.json such as comments or trailing commas (actually, it's stripping comments - but trailing commas still appear to be an issue)

[ZSkycat]

@blakeembrey

I have a suggestion for how this could be detected

export function isRegistered(){
    return require.extensions['.ts'] != undefined || require.extensions['.tsx'] != undefined;
}

Maybe I should create PR?

[blakeembrey]

That’s how this module works but it doesn’t tell you if it’s already been registered - anything could be on those extensions already. A global would be the most appropriate way to solve this, but not sure where it should live.