Building removes a variable and its implementations + conditional statements using it

[robskaar]

Describe the bug

I am building a test runner script for my webapp in react.
It's an iterative "looping" scripts that runs the webapp over multiple times.
The first 3 times i need to do a quick test, then test should be long afterwards.

When i run the project npm run dev - basically just "vite" command, the first 3 test are running quickly, then the rest are slow - so my TS files seem to work in development.

When i run build the project npm run build - "tsc && vite build" command, and then host that webapp and open it, the every test is quick - not like in development.

• i see in my build js file that i cannot find some of the variables associated with my quick test evaluation nor the implementation of those missing variables (e,g. used in shorthand if/else operations like const quickTestTime = isQuickTest ? (5 * 60 * 1000) / 15 : 5 * 60 * 1000 but it then shows const quickTestTime = (5 * 60 * 1000) / 15 instead so the "isQuickTest" variable gets stripped away and the logical operations for it.

if i run tsc myfile.ts everything compiles til JS perfectly. if i run npm run dev, i get no runtime errors & the ts seems to work perfectly as expected. theres no build errors but for whatever reason something that works in production and that tsc can compile perfectly to JS gets stripped out from vite build process - leaving me no errors to investigate, so i just used a shitload of time finding this issue.

I know why it could make sense for the build process to strip it out - it evaluates to true || 1, so the variable as well as the condition is "not needed" from a programatic view, but from a developer / intended view in my opinion it is verry bad DevEx to strip out conditional statements with no warnings

Reproduction

https://stackblitz.com/edit/vitejs-vite-9y8hev2g?file=build%2Fassets%2Findex-CZk89dWW.js

Steps to reproduce

in the reproduction site,
npm uninstall tsc
npm install typescript
npm run build

view the build js file and see that RunTestFails() function is missing quicktesttime var and implementations

System Info

System:
    OS: Linux 5.0 undefined
    CPU: (8) x64 Intel(R) Core(TM) i9-9880H CPU @ 2.30GHz
    Memory: 0 Bytes / 0 Bytes
    Shell: 1.0 - /bin/jsh
  Binaries:
    Node: 20.19.1 - /usr/local/bin/node
    Yarn: 1.22.19 - /usr/local/bin/yarn
    npm: 10.8.2 - /usr/local/bin/npm
    pnpm: 8.15.6 - /usr/local/bin/pnpm
  npmPackages:
    vite: ^7.0.0-beta.1 => 7.0.0-beta.2

Used Package Manager

npm

Logs

No response

Validations

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• The provided reproduction is a minimal reproducible example of the bug.

[sapphi-red]

When i run the project npm run dev - basically just "vite" command, the first 3 test are running quickly, then the rest are slow - so my TS files seem to work in development.

I don't understand why it works in development. const isQuickTest = testCount === '1' || testCount === '2' || testCount === '3' || null || undefined || 1 || 2 || 3; will always evaluate to true unless you are using Function::toString + eval. So replacing it with true is normally safe. All tools in the ecosystem I'm aware of does not support Function::toString (as it's impossible to support them).
Why does it work in dev? Would you make a reproduction that I can try that the dev is working?

[github-actions]

Hello @robskaar. Please provide a minimal reproduction using a GitHub repository or StackBlitz. Issues marked with needs reproduction will be closed if they have no activity within 3 days.

[robskaar]

Im sorry, i confused myself and you, in development theyre all running quicktest (because it of course evaluates to true).
My issue is that by "working in development" i really meant that i at no point in time get a warning for my improper code, ts compiles it to JS without removing the unneeded logical operation, i dont get an IDE warning, theres no log in browser console and there is no build time error, so everywhere that could assist me in finding the issue of my improper written conditional logic, doesnt.

I do understand i could have some linting rules that may catch this issue for me in the IDE, but that set by peoples preferences, the browser console cant log me anything there isnt thrown, Typescript shouldnt strip anything out just show it as compiled JS, but as the build step strips out code that i expect to be there, for me it seems like a conflict that should throw either a error or a warning.
if i did
var test = true;
and never used it - theres no intent behind the variable so i understand why that would be removed.
but if i do
var auth = testCount === '1' || testCount === '2' || testCount === '3' || null || undefined || 1 || 2 || 3; const doSomethingWithAuth = (auth: number | true) => { if (auth) { console.log('auth'); } else { console.log('noauth'); } }; doSomethingWithAuth(auth);
this would all get stripped out of my code when i build and become:
const doSomethingWithAuth = (auth2) => { { console.log("auth"); } }; doSomethingWithAuth();
new stackblits link with this example aswell: https://stackblitz.com/edit/vitejs-vite-9y8hev2g?file=build%2Fassets%2Findex-DqMvlBOD.js
So my auth intent would be removed without me recieving any errors -> meaning there could now be auth issues in my application because my code isnt working and i get no errors for it, this of course would most likely be picked up by me while developing it, but lets consider this piece of code being modified like this by some junior dev 3 years later to always evaluate true, and him never getting an error for, but the build tool actually knowing theres "unused developer intent".

So as a developer i really would expect something like the build tool removing my logical operations (and thereby some possible intent in my application flow) in build time should give a build error/warning at least, enabling me to easily find my issue and correct it.

i understand that this may be opinionated, but for me atleast i wrote buggy code that ran in without me ever recieving an error for it, but with a build step that could have informed me about my "unused intent" that it stripped away, enabling me to save about 5 hours of debugging because i didnt have the relevant Linter rule that is the only other option to catch this bug in my code - but i rather see the responsibility of informing of this on the build tool than the linter, as linters are more setup by teams preferences, but build tools are more often used with default configs.
e.g. like building with an variable thats always true warns in Visual studio (where they do not strip out my code)

So at the end of the day id expect a warning after building "The result of expression is always X, operational decisions in the Y flow have been removed, consider removing Y flow or fixing expression"

[sapphi-red]

Even though you think it's an "improper" code, the bundler cannot treat that it's a "improper" code. It is valid to have a code generated that contains const something = someCondition || 1, especially when you use constant replacements.
Vite does not know whether the code is written by hand or generated by something. So Vite does not know whether it's "improper".