[Bug]: Huge performance drop when "allowJs": true is enabled in tsconfig

[LoganTann]

Version

at least since 0.3.3, tried in 0.5.3 and looks even worse (no proofs though)

Steps to reproduce

Minimal reproduction repository : https://github.com/LoganTann/tsjest-performance-issue-reproduction

• Clone the repo
• npm i
• npm run test

Expected behavior

• Jest should take 8 seconds to load the dependency, not more than 10 minutes.

Actual behavior

• Simple test is taking an extremely long time (> 10min) to load the dependency

• Disabling the {allowJs} property in the tsconfig file will make the test run under 8 seconds. However, jest will print over 1400 warnings (executed grep " file to compile while" output_nojs.log | wc -l).

• After a successful run, later tests are expected to be much faster thanks to the caching.

Debug log

Interrupted execution, took too much time.

Additional context

• Running in WSL 2, inside the ~/reproduction/ folder
• No performance issues in the back-end, both in dev and prod. The code compiles and runs fine.
• SWC + vitest is not affected by this issue, but I cannot migrate the whole app in another tech stack.

Performance measures I did five months ago (under 0.3.3)

cross-env NODE_ENV=test jest -runInBand --maxWorkers=1 SalesforceConnections.controller.spec.ts --no-cache

Imports : 
    HttpService: 62226 ms (@nestjs/axios)
    BadRequestException: 1 ms
    ConfigModule: 4736 ms (@nestjs/config)
    Test: 22756 ms (@nestjs/testing)
    TypeOrmModule: 133894 ms (@nestjs/typeorm)
    Repository: 0 ms
    configuration: 9 ms
    RequestUserModel: 8 ms
    CryptoService: 33 ms
    entities: 153 ms
    FixtureService: 109 ms
    TenantEntity: 0 ms
    TenantModel: 5 ms
    TenantService: 34 ms
    UserEntity: 0 ms
    SalesforceConnectionsController: 183104 ms (imports the same)
    SalesforceInstanceDetailsResponseDto: 0 ms
    SalesforceConnectionEntity: 0 ms
    SalesforceInstanceEntity: 0 ms
    SalesforceConnectionService: 0 ms
    SalesforceInstanceService: 0 ms
    SalesforceOauthService: 0 ms
    ConnectivityStatus: 1 ms
Test module compilation :
    Compile: 11315 ms

Environment

System:
    OS: Linux 5.15 Ubuntu 22.04.3 LTS 22.04.3 LTS (Jammy Jellyfish)
    CPU: (8) x64 11th Gen Intel(R) Core(TM) i7-1165G7 @ 2.80GHz
  Binaries:
    Node: 18.19.0 - ~/.nvm/versions/node/v18.19.0/bin/node
    npm: 10.2.3 - ~/.nvm/versions/node/v18.19.0/bin/npm
  npmPackages:
    jest: ^29.7.0 => 29.7.0

[LoganTann]

Related: #1115

[LoganTann]

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.

[ahnpnl]

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

[LoganTann]

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.

[ahnpnl]

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)

[LoganTann]

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

[LoganTann]

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

[ahnpnl]

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.

[LoganTann]

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.

[ahnpnl]

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

[LoganTann]

Created a documentation PR to fix this : #4567