@rollup/plugin-babel causes SOURCEMAP_ERROR while reporting THIS_IS_UNDEFINED from unmapped generated helper code #2005

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1. RagedPanda commented on Oct 22, 2026

   

   RagedPanda

   on Oct 22, 2026

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   Dug into this a bit. I don't think plugin-babel is doing anything wrong here. The warning comes from Rollup core.

   Babel reads index.js.map (inputSourceMap is on by default) and merges it, so its output map has nothing on line 1, same as the input:

   input map:   ;AAAA,OAAO,MAAM,KAAK,GAAG,CAAC
   babel map:   ;AAAA,OAAO,MAAMA,KAAK,GAAG,CAAC
   

   When Rollup then raises THIS_IS_UNDEFINED at 1:17, addLocationToLogProps calls getOriginalLocation on the sourcemap chain. That throws as soon as it hits a line with no segments, and the catch turns it into SOURCEMAP_ERROR. Bare Rollup never loads index.js.map, so there's no chain to walk and you only get the original warning.

   Rollup has a test asserting exactly this behavior (test/function/samples/cannot-resolve-sourcemap-warning, a transform that returns mappings: '' and expects SOURCEMAP_ERROR before THIS_IS_UNDEFINED), so it looks intentional. If it should be quieter for valid-but-unmapped helper code, that's probably a change for rollup/rollup rather than this plugin.

   Workarounds in the meantime, both checked against your repro:

   • Filter it in onwarn:

     onwarn(warning, warn) {
       if (warning.code === 'SOURCEMAP_ERROR') return;
       warn(warning);
     }

   • Or pass inputSourceMap: false to babel(). The warnings go away, but your output map then points at the compiled index.js instead of original.ts, so I'd only do that if you don't need the chain.