Detecting TypeScript support

[B4nan]

🚀 Feature Proposal

When I run the tests with ts-jest preset, I am able to run/require TS files in the code. I would like to detect this fact on runtime, e.g. via environment variable like TS_JEST=true.

Motivation

I am author of MikroORM. It allows to discover entities either via references (entities: [User]) or via paths (entities: ['./entities']). To use the folder based discovery, I need to operate on the TS files when running via ts-node or ts-jest, as I am attaching some properties to the entity prototype and therefore need to work with TS file if that is what will be later used on runtime (in other words, if ts-node/ts-jest is used, I need to discover TS files from src, otherwise JS files from dist).

To deal with this, I have two different config options for entity paths, one for JS files, one for TS files. If I am able to detect typescript support (e.g. ts-node) I am using the TS one.

While there are several ways to detect if we are under ts-node environment (e.g. using !!process[Symbol.for('ts-node.register.instance')]), I failed to find one to detect if we are running via jest with ts-jest preset.

Example

Barebones example would be:

function discoverEntities(path: string) {
  const files = await globby(path);

  return files.map(path => {
    // for simplicity let's expect default export
    return require(path).default;
  })
}

if (process.env.TS_JEST) {
  const entities = discoverEntities('./src/entities/*.ts');
} else {
  const entities = discoverEntities('./dist/entities/*.js');
}

[ahnpnl]

You can achieve this easily by setting process.env.TS_JEST directly in your jest config file.

It is not possible for us to inject process.env because ts-jest is a Jest transformer which is triggered by Jest runtime. Anything we set in process will not impact your codes at all since we run inside Jest worker.

ts-jest is different from ts-node from architecture point of view because ts-node is a standalone entity while we rely on Jest architecture. If you want to set anything which impacts cross processes and global, you need to do it in jest config file.

[B4nan]

Well, but I am also running the code inside the jest worker. Tried to add process.env.TS_JEST = '1' to the dist/index.js file and I am able to read that value from my code where I try to detect it.

edit: mmmm but that is now executed on require time, right

[ahnpnl]

Oh that looks good. PR is welcome :)

[B4nan]

I am not sure if doing it on require time is a good idea, might be ok. When I tried to put it to various places in the transofer like to TsJestTransformer.prototype.process, it failed. But when I put it to the constructor, it works fine, so I guess that will be the ideal place for this.

Will PR it in a minute!

[ahnpnl]

You can either put in index.ts or constructor of the transformer class, both should work the same I expect.

[B4nan]

Here you go #2717, if you can think of some nice way to test it, I am all ears.