svelte: Error in preprocessor sourcemapping: "Cannot read property 'length' of undefined"

Describe the bug The new preprocessor sourcemapping feature seems to sometimes cause errors (CC @halfnelson @milahu)

Logs

> sapper-postcss-template@2020.11.18 build /Users/babichjacob/Repositories/sapper-postcss-template
> cross-env NODE_ENV=production sapper build --legacy

> Building...

[!] (plugin svelte) TypeError: Cannot read property 'length' of undefined
/Users/babichjacob/Repositories/sapper-postcss-template/src/routes/index.svelte
TypeError: Cannot read property 'length' of undefined
    at sourcemap_add_offset (/Users/babichjacob/Repositories/sapper-postcss-template/node_modules/svelte/compiler.js:22087:23)
    at get_replacement (/Users/babichjacob/Repositories/sapper-postcss-template/node_modules/svelte/compiler.js:28575:10)
    at /Users/babichjacob/Repositories/sapper-postcss-template/node_modules/svelte/compiler.js:28635:21
    at async Promise.all (index 0)
    at async replace_async (/Users/babichjacob/Repositories/sapper-postcss-template/node_modules/svelte/compiler.js:28551:52)
    at async preprocess_tag_content (/Users/babichjacob/Repositories/sapper-postcss-template/node_modules/svelte/compiler.js:28618:22)
    at async preprocess (/Users/babichjacob/Repositories/sapper-postcss-template/node_modules/svelte/compiler.js:28644:10)
    at async Object.transform (/Users/babichjacob/Repositories/sapper-postcss-template/node_modules/rollup-plugin-svelte/index.js:100:23)
    at async ModuleLoader.addModuleSource (/Users/babichjacob/Repositories/sapper-postcss-template/node_modules/rollup/dist/shared/rollup.js:18289:30)
    at async ModuleLoader.fetchModule (/Users/babichjacob/Repositories/sapper-postcss-template/node_modules/rollup/dist/shared/rollup.js:18345:9

To Reproduce git clone git@github.com:babichjacob/sapper-postcss-template.git

Upgrade to svelte 3.30 and rollup-plugin-svelte 7 following the upgrade guide (example of that here: https://github.com/sveltejs/sapper-template/pull/289)

sourcemap = false -> no error sourcemap = “inline” -> that error sourcemap = true -> that error

Severity Several people have reported this issue on Discord. It seems to happen relatively frequently

About this issue

Original URL
State: closed
Created 4 years ago
Reactions: 11
Comments: 22 (15 by maintainers)

Most upvoted comments

It was explained: https://bytearcher.com/articles/semver-explained-why-theres-a-caret-in-my-package-json/

That package.json specifies ^3.17.3 which means get the latest 3.x release, which is 3.30. The actual version used is specified in the package-lock.json. Try grep -r -A2 svelte package-lock.json

The method this bug refers to did not exist until 3.30, which means you’re either using 3.30 or encountering a different issue.

Damn I didn’t known it did that, thank you and sorry for being arrogant 😕 It worked with the ’~‘, thank you

jacobdalamb on Nov 26, 2020

My analysis so far: The PostCSS processor in svelte-preprocess is returning a SourceMapGenerator instead of whatever the svelte compiler is expecting it to return. This SourceMapGenerator is coming directly from the postcss package. I’m not sure where exactly the SourceMapGenerator is supposed to turn into an actual source map, but that seems to be the missing step AFAICT.

Update: Looks like just changing the postcss transformer isn’t sufficient.

This is certainly not the exact right fix, but adding this in the Svelte compiler.js at line 28636, just before the call to get_replacement allows everything to build without errors:

if (processed.map?._mappings) {
    processed.map = processed.map.toString();
}

dimfeld on Nov 25, 2020

It was explained: https://bytearcher.com/articles/semver-explained-why-theres-a-caret-in-my-package-json/

That package.json specifies ^3.17.3 which means get the latest 3.x release, which is 3.30. The actual version used is specified in the package-lock.json. Try grep -r -A2 svelte package-lock.json

The method this bug refers to did not exist until 3.30, which means you’re either using 3.30 or encountering a different issue.

benmccann on Nov 26, 2020