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Fix sourcemaps failing on refresh #1755

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merged 2 commits into from
Jul 22, 2018
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Fix sourcemaps failing on refresh #1755

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480036a
fix hmr sourcemaps
DeMoorJasper Jul 18, 2018
36fe7a3
Manual clone for perf
devongovett Jul 22, 2018
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14 changes: 8 additions & 6 deletions src/SourceMap.js
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Original file line number Diff line number Diff line change
Expand Up @@ -79,12 +79,14 @@ class SourceMap {
}

addMapping(mapping, lineOffset = 0, columnOffset = 0) {
mapping.generated = {
line: mapping.generated.line + lineOffset,
column: mapping.generated.column + columnOffset
};

this.mappings.push(mapping);
this.mappings.push({
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Not familiar with this API, but does this mapping have a .name? I see it used in addConsumerMapping, but not this one.

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Ow it should, thanks for noticing

source: mapping.source,
original: mapping.original,
generated: {
line: mapping.generated.line + lineOffset,
column: mapping.generated.column + columnOffset
}
});
}

addConsumerMapping(mapping, lineOffset = 0, columnOffset = 0) {
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@loganfsmyth loganfsmyth Jul 26, 2018 •
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FYI, the handling below here seems questionable. Mappings with no original location are still important because they terminate the end of a conceptual range over the generated file by saying that a generated location points at nothing. By discarding mappings with no original location, you're essentially force-extending every generated mapping to instead extend to the end of the line/next original-location mapping.

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Not sure why this would be an issue, if code is generated it shouldn't have a mapping right?

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Not quite. Sourcemaps are conceptually ranges over the generated code, but the actual mappings passed from the consumer are single points of data, from one generated line/col to an original line/col (optionally).

It's the fact that the original part is option that is critical here, and that is getting lost with the check in

     if (
       !mapping.source ||
       !mapping.originalLine ||
       (!mapping.originalColumn && mapping.originalColumn !== 0)
     ) {
       return;
     }

Imagine a generated piece of code like this:

console.log

If I want to map just the console to something in the original JS file, setting no original location on the .log part, I'd generate two mappings. One would say

{
  generated: {
    line: 0,
    column: 0,
  },
  original: {
    line: 0,
    column: 0,
  },
  source: "input.js",
  name: "console",
}

to say that the start of the word "console" (line 0, column 0) maps to the original file, also at line 0 column 0.

So far so good, but nothing sets an end position, so that mapping on its own would also apply to the .log part.

That's where non-source mappings come in. You'd also include a mapping as

{
  generated: {
    line: 0,
    column: 7,
  },
  original: null
  source: null,
  name: null,
}

to say that, starting at line 0, column 7, the code maps to nothing in the original file.

By discarding that information, you've essentially made the original line 0 column 0 mapping apply to the whole of console.log.

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Ow I never saw that as an issue but I'll definitely look into it.
Now that you've explained it, I can see it could and will cause issues.

Thank you

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No problem, I'm happy to answer any other sourcemap questions if you have them. Babel has had it's fair share of questionable mapping logic over the ears too.

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