`__NO_SIDE_EFFECTS__` does not support functions from outside modules

[ocavue]

Given the following input

// main.js
import { g } from './g.js';

/* @__NO_SIDE_EFFECTS__ */
function f(a) { console.log('function_f' + a) }

function h(a) { console.log('function_h' + a) }

f('removeThisCall_1')
g('removeThisCall_2')
h('keepThisCall_3')

// g.js
/* @__NO_SIDE_EFFECTS__ */
function g(a) { console.log('function_g' + a) }

export { g }

I expected g('removeThisCall_2') to be removed in the final output, but esbuild still keeps it.

• Expected behavior (rollup):

  https://rollupjs.org/repl/?version=4.53.0&shareable=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

• Acutall behavior (esbuild):

  https://esbuild.github.io/try/#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

[hyrious]

Yes, this was mentioned in the changelog when esbuild initially supports that annotation: https://github.com/evanw/esbuild/blob/main/CHANGELOG-2023.md#:~:text=preserve%20them%20in%20output%20files

That is to say, esbuild currently only tree-shakes those statements in the same file. No cross-file tree-shaking on these annotations happened for now. Note that esbuild also preserves these statements (if you do not minify), which makes it possible for other tools like Rollup to further optimize the output.

[ocavue]

Ah, thank you for pointing it out. The following quoted text from the CAHNGELOG indeed describes the implementation well.

Support /* @__NO_SIDE_EFFECTS__ */ comments for functions

...

Note that this doesn't necessarily mean esbuild will treat these calls as having no side effects, as esbuild's parallel architecture currently isn't set up to enable this type of cross-file tree-shaking information (tree-shaking decisions regarding a function call are currently local to the file they appear in). If you want esbuild to consider a function call to have no side effects, make sure you continue to annotate the function call with /* @__PURE__ */ (which is the previously-established convention for communicating this).