`//# sourceMappingURL=...` doesn't have to be on the last line

[nicolo-ribaudo]

Firefox, Chrome, Safari and Node.js all seem to allow it, but the spec is not explicit about it.

[hybrist]

IIRC it goes even further and they typically allow additional lines that aren't empty after the comment? I think we have some code that depends on the ability to insert an additional marker comment after the line with the source map comment.

[nicolo-ribaudo]

I just checked and we do the same in Babel

[hybrist]

Potentially related: Allowing additional lines immediately means that there's ambiguity because it's no longer well defined which source map comment is supposed to win if there's multiple. I'm not sure if consumers consistently pick the last one.

[connor4312]

consumers consistently pick the last one

Consumers should, because bundles with multiple //# sourceMappingURL are pretty common as output from bundlers that bundle previously-compiled files without additional introspection (I think I saw this in Webpack)

[kamilogorek]

Sentry appends //# debugId=00000000-0000-0000-0000-000000000000 after sourceMappingURL right now.
As for detection, we used to fallback to scanning last 50 (if I recall correctly?) lines of source code in case we didn't find it at the very end. Now it always scans through all lines.

[nicolo-ribaudo]

I just checked with this test case:

"use strict";

throw new Error("Hello world!");
//# sourceMappingURL=data:application/json;base64,eyJ2ZXJzaW9uIjozLCJmaWxlIjoiIiwic291cmNlUm9vdCI6IiIsInNvdXJjZXMiOlsiaHR0cDovL2xvY2FsaG9zdDo0NTQ1L3J1bi9pbmxpbmVfanNfc291cmNlX21hcF8yLnRzIl0sInNvdXJjZXNDb250ZW50IjpbIjErMTtcbmludGVyZmFjZSBUZXN0IHtcbiAgaGVsbG86IHN0cmluZztcbn1cblxudGhyb3cgbmV3IEVycm9yKFwiSGVsbG8gd29ybGQhXCIgYXMgdW5rbm93biBhcyBzdHJpbmcpO1xuIl0sIm5hbWVzIjpbXSwibWFwcGluZ3MiOiI7QUFBQSxDQUFDLEdBQUMsQ0FBQyxDQUFDO0FBS0osTUFBTSxJQUFJLEtBQUssQ0FBQyxjQUErQixDQUFDLENBQUMifQ==
console.log("AAA")
// some random comment

Firefox, Chrome, Safari, Node.js and Babel all properly detect the source map. It looks like there is indeed no restriction at all about where the //# sourceMappingURL= should be.

[nicolo-ribaudo]

Same thing with this:

"use strict";

throw new Error("Hello world!");
; //# sourceMappingURL=data:application/json;base64,eyJ2ZXJzaW9uIjozLCJmaWxlIjoiIiwic291cmNlUm9vdCI6IiIsInNvdXJjZXMiOlsiaHR0cDovL2xvY2FsaG9zdDo0NTQ1L3J1bi9pbmxpbmVfanNfc291cmNlX21hcF8yLnRzIl0sInNvdXJjZXNDb250ZW50IjpbIjErMTtcbmludGVyZmFjZSBUZXN0IHtcbiAgaGVsbG86IHN0cmluZztcbn1cblxudGhyb3cgbmV3IEVycm9yKFwiSGVsbG8gd29ybGQhXCIgYXMgdW5rbm93biBhcyBzdHJpbmcpO1xuIl0sIm5hbWVzIjpbXSwibWFwcGluZ3MiOiI7QUFBQSxDQUFDLEdBQUMsQ0FBQyxDQUFDO0FBS0osTUFBTSxJQUFJLEtBQUssQ0FBQyxjQUErQixDQUFDLENBQUMifQ==
console.log("AAA")
// some random comment

it looks like in practice the comment doesn't even have to be on its own line, it can be in the same line together with other code

[nicolo-ribaudo]

This is very fun:

"use strict";

throw new Error("Hello world!");`
//# sourceMappingURL=data:application/json;base64,eyJ2ZXJzaW9uIjozLCJmaWxlIjoiIiwic291cmNlUm9vdCI6IiIsInNvdXJjZXMiOlsiaHR0cDovL2xvY2FsaG9zdDo0NTQ1L3J1bi9pbmxpbmVfanNfc291cmNlX21hcF8yLnRzIl0sInNvdXJjZXNDb250ZW50IjpbIjErMTtcbmludGVyZmFjZSBUZXN0IHtcbiAgaGVsbG86IHN0cmluZztcbn1cblxudGhyb3cgbmV3IEVycm9yKFwiSGVsbG8gd29ybGQhXCIgYXMgdW5rbm93biBhcyBzdHJpbmcpO1xuIl0sIm5hbWVzIjpbXSwibWFwcGluZ3MiOiI7QUFBQSxDQUFDLEdBQUMsQ0FBQyxDQUFDO0FBS0osTUFBTSxJQUFJLEtBQUssQ0FBQyxjQUErQixDQUFDLENBQUMifQ==
// `

Chrome/Firefox/Safari/Babel and Node.js (but only for CJS) properly ignore the "comment", while Node.js ESM accepts it and applies the source map

[nicolo-ribaudo]

With regards to CSS:

• Safari, Chrome and Firefox all detect the source map in

  body .test {
    font-size: 14px;
    line-height: 28px; }
  
  body .text {
    color: green; }
    body .test::after {
      content: "";
      background-color: white; }
  
  body .test {
    padding: 5px; }
  
  a::before { /*# sourceMappingURL=sourcemap-css-absolute.map */ }

• Safari and Chrome mistakenly detect a source map in

  body .test {
    font-size: 14px;
    line-height: 28px; }
  
  body .text {
    color: green; }
    body .test::after {
      content: "";
      background-color: white; }
  
  body .test {
    padding: 5px; }
  
  a::before { content: "/*# sourceMappingURL=sourcemap-css-absolute.map */" }

[mitsuhiko]

I'm not sure what browsers currently do but some tools (we included) only honor a few lines from top to bottom of file for efficiency reasons. I think the question here might be if the specification can better explain what exactly the expectations are in what tools are supposed to do. It's also common for source mapping URL comments to appear more than once and which one should be picked.

My recommendation would be to request a minimum number of lines or bytes to be considered from both start and end the file and to ask tools to prioritize the last one in the file over earlier ones. (Assuming that this is the order that browers pick today)