What is the point of Jest's createTransformer method in its *Transformer interface?

Question

I have been looking into/debugging code transformation related issues in Jest for the last day and a recurring theme is that the SyncTransformer#createTransformer method is a constant source of surprise and it is not really documented why it exists.

The SyncTransformer interface only has a single field one has to implement: process. But it seems that if one implements createTransformer those other methods will not be used: instead Jest seems to create a new transformer using createTransformer, which caused me to lose a few hairs until I figured what was going on. This behaviour is not documented either.

The babel-jest source for Jest 27.

Answer 1

I filed a documentation bug issue with Jest after I could see that the behavior was not mirrored in the docs, which I subsequently fixed by updating the types and docs for code transformation.

The rules for this is basically like follows:

• if createTransformer exists as an export, then jest-transform will use that to create a transformer dynamically and not use any of the other exports
• if the transformer is imported using import (and not require) it will try to use processAsync and fall back to process if the async version does not exist