Dead code elimination with constant return functions

[schardev]

Summary

I noticed that dead code elimination does not work as expected when using functions with constant return values. Here's an example to illustrate the issue:

const isProd = () => process.env.NODE_ENV !== 'development';

// This conditional check will NOT be removed by Next.js during the build process,
// even when NODE_ENV is set to 'development'. The `isProd` function check remains in the output.
if (isProd()) {
    const prodValue = 'value';
}

// However, this conditional check WILL be removed by Next.js during the build process,
// as it directly evaluates the environment variable.
if (process.env.NODE_ENV !== 'development') {
    const prodValue = 'value';
}

It looks like wrapping the check in a function prevents Next.js/SWC from optimizing it away. Is there any workaround for this, or is it something that would require internal changes to Next.js, like a double-pass build to eliminate dead code?

Additional information

No response

Example

No response

[icyJoseph]

What Next.js version do you have.

Looks like up to certain SWC versions, this didn't work so well. Here for example ~ upgrading to 1.3.4 fixes it

I tried in the latest stable Next.js with:

const isDev = () => process.env.NODE_ENV === 'development';

// This conditional check will NOT be removed by Next.js during the build process,
// even when NODE_ENV is set to 'development'. The `isProd` function check remains in the output.
if (isDev()) {
  const devValue = 'value';
  console.log('1337-420', devValue);
}

On a page, and that code didn't make it to production.

Maybe I am doing something wrong, this kind of dead code elimination has always been a bit fragile. Could you share more details?

[schardev]

The trigger is that you are doing these checks within a function

True. Just verified it. Majority of the times the check would be within a function though 😅

I guess, at runtime, one could argue that these functions could change their output?

I do agree with that to a certain degree. However, since the return type of these functions remains constant, I assumed the bundler or minifier would be smart enough to optimize it away. Either way, this seems like a valid case for a bug report.

Thanks for helping to track this down. Really appreciate it.

[icyJoseph]

I see that another plugin, for babel in this case, also misses it...

If I take the output of SWC and take it to Terser, is does minify it finally... I wonder if its an issue with the number of passes and such

[schardev]

I wonder if its an issue with the number of passes and such

Yeah. Feels like it.

[icyJoseph]

Though increasing the number of passes on the SWC playground does not do the trick

[schardev]

A more simpler example, behaves the same on babel and swc:

babel

swc

[schardev]

Ref: swc-project/swc#10448

[schardev]

This has been fixed now in SWC v1.12.1. Now waiting for next.js to update their SWC version, though I'm not sure how to check which SWC version next uses.