[Bug]: Huge performance drop when "allowJs": true is enabled in tsconfig about ts-jest HOT 10 OPEN

Related: #1115

from ts-jest.

Found a workaround, suggested by the warning message when you disable "allowJs" :

• Remove "compilerOptions.allowJs": true in your tsconfig file
• Edit the transform regex to select only typescript files with ts-jest :

'transform': {
    '^.+\\.ts$': 'ts-jest', // << selects only ts. It was previously '^.+\\.(j|t)s$'
},

This is still an issue. This needs to be documented, as I had this issue since at least january and I had to investigate since I cleared my jest cache.

from ts-jest.

A bit background for this issue: ts-jest internally uses TypeScript compiler API to transform ts/js file into js file.

In general, setting allowJs to true is necessary if one wants the TypeScript compiler API to load and process JavaScript files in addition to TypeScript files when transforming files. This leads to the result that more files are loaded into memory which can blow up the machine.

Currently there are 2 workarounds:

• Use https://kulshekhar.github.io/ts-jest/docs/getting-started/options/isolatedModules
• Or your solution which tells ts-jest not to process js file

With the release of typescript 5.5, now we can actually use other transpilers like esbuild or swc to process js files + opt in type checking from new v5.5 API

from ts-jest.

Good to know, thanks for the answer @ahnpnl ! Isolated modules did not solve the issue for me, but removing js files from ts-jest fixed it.

I just created another issue here : thisismydesign/nestjs-starter#468
It is very likely the previous developer in my project took the ts-config file in this template repo.

from ts-jest.

What is the issue with isolatedModules that you had? It supposed to be faster with true value (this is not tsconfig option btw, the name is confusing and we will rename it)

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The reproduction repo has the isolatedModules set to true and my measurements takes account of this.

https://github.com/LoganTann/tsjest-performance-issue-reproduction/blob/main/jest.config.js

from ts-jest.

But the slowness is only due to js files being included in the transformer regex. isolatedModules does not seems to have any issue.

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I see. Yes in this case Jest will give both js and ts to ts-jest to transform which explained the issue.

Theoretically if using fast transpiler like swc, it should be better. However, the recommendation is still only transform what is needed.

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The recommendation is still only transform what is needed.

For sure ! In fact, to solve this issue, only a simple warning in the documentation would be required. We cannot do much to solve this, as in order to get this behavior, you need a configuration that is not recommended.

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I agree, I will update documentation to have a note about this. Thanks for the suggestion!

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