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minimizer-webpack-plugin depends on @jridgewell/trace-mapping only to compose maps across chained minimizers and to point a minifier error back at its original location. Composing is already SourceMapSource with an inner source map; the lookup was missing. This adds it, built on the existing readMappings decoder:
const{ util }=require("webpack-sources");util.sourceMap.originalPositionFor(map,1,4);// → { source: "a.js", line: 1, column: 4, name: "foo" }, or null when nothing maps there
Same convention as trace-mapping: lines from one, columns from zero, closest mapping at or before the column. Accepts the map as an object, JSON string or Buffer, and joins sourceRoot like getSource.
What kind of change does this PR introduce?
feat
Did you add tests for your changes?
Yes, test/sourceMap.js (exact and nearest column, no mapping, segment without a source, sourceRoot, string/Buffer input, a map OriginalSource produced) and test/package-entry.js. 100% line and branch coverage of the new file.
Does this PR introduce a breaking change?
No.
If relevant, what needs to be documented once your changes are merged or what have you already documented?
Documented in README.md (util.sourceMap).
Use of AI
Written with Claude Code from my requirements; I reviewed the API and the tests.
Added util.sourceMap.originalPositionFor to look up the original source position for a generated line and column. It accepts a source map object, JSON string, or buffer, and returns the closest mapping at or before the requested column—or null if none exists.
Documentation
Documented the lookup’s line and column numbering, sourceRoot handling, and whole-map decoding behavior for one-off lookups.
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Inline comments:
Review comments at @lib/helpers/sourceMap.js:
- Line 63: **Return `null` when a mapping's source index is out of range.**
`originalPositionFor` passes the decoded source index to `getSource` without
checking it against `map.sources`. For an out-of-range index, `getSource`
returns `undefined`. The function then returns an object whose `source` is
`undefined`, although its declared type is `string`. Return `null` for this
case, as the function already does for mappings that carry no source.
<details>
<summary>🐛 Suggested fix</summary>
```diff
- if (sourceIndex < 0) return null;
+ if (sourceIndex < 0 || sourceIndex >= map.sources.length) return null;
```
</details>
- Line 66: **Bound-check `names` before indexing.**
A map whose named segment points past the end of `names` returns `name:
undefined`. The declared type is `string | null`. Add a length check so
out-of-range indices return `null`.
<details>
<summary>🐛 Suggested fix</summary>
```diff
- name: nameIndex >= 0 && map.names ? map.names[nameIndex] : null,
+ name:
+ nameIndex >= 0 && map.names && nameIndex < map.names.length
+ ? map.names[nameIndex]
+ : null,
```
</details>
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lib/helpers/sourceMap.js
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.changeset/source-map-original-position.md
README.md
lib/helpers/sourceMap.js
lib/index.js
test/package-entry.js
test/sourceMap.js
types.d.ts
types/helpers/sourceMap.d.ts
types/index.d.ts
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Walkthrough
The change adds util.sourceMap.originalPositionFor to find the closest mapping at or before a generated column on a specified line. The helper accepts a source map object, JSON string, or Buffer. It returns the original source position and optional name, or null when no mapping applies. The change also exposes the helper in the runtime and TypeScript APIs and adds tests, README documentation, and a changeset.
Priority: Normal
Merge Risk: Low · previous review fddfb
The lookup is mergeable with bounded follow-up: malformed maps can produce undefined fields instead of the documented results. Adding index checks would close these edge cases.
Pre-merge checks: 5
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Add a lookup test that selects a nonzero source index.
The originalPositionFor fixture defines one source, sources: ["a.js"]. With one source, index 0 is the only valid index. A lookup that returned sources[0] instead of sources[sourceIndex] would give the same result, so the tests would still pass. The helper passes the decoded source index to getSource, so the current behavior is correct. The gap is in test coverage, not in production code.
Add a second entry to sources in the fixture. Add a segment that decodes to source index 1. Assert that the lookup returns that second source.
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.changeset/source-map-original-position.md
README.md
lib/helpers/sourceMap.js
lib/index.js
test/package-entry.js
test/sourceMap.js
types.d.ts
types/helpers/sourceMap.d.ts
types/index.d.ts
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System calls cannot be simulated consistently and are left out of the measure. The reported value understates the real cost. Switch to the Walltime instrument to measure them.
Some benchmarks with significant performance changes were compared across different runner types, so the changes might not come from the code. Run base and head on the same runner type, or use CodSpeed Macro Runners for more stable results.
✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.11%. Comparing base (99067d0) to head (420436a). ⚠️ Report is 3 commits behind head on main.
CodSpeed Performance Analysis is red on one benchmark, cached-source memory: warm sourceAndMap() returns cached references (8 B → 992 B), which isn't from this PR:
This PR only adds lib/helpers/sourceMap.js, loaded lazily through util.sourceMap; nothing on the CachedSource path requires it.
CodSpeed's comparison lists this benchmark under an environment difference between base and head: Node 24.21.0 → 24.22.1 (V8 node.53 → node.58).
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Summary
minimizer-webpack-plugindepends on@jridgewell/trace-mappingonly to compose maps across chained minimizers and to point a minifier error back at its original location. Composing is alreadySourceMapSourcewith an inner source map; the lookup was missing. This adds it, built on the existingreadMappingsdecoder:Same convention as
trace-mapping: lines from one, columns from zero, closest mapping at or before the column. Accepts the map as an object, JSON string orBuffer, and joinssourceRootlikegetSource.What kind of change does this PR introduce?
feat
Did you add tests for your changes?
Yes,
test/sourceMap.js(exact and nearest column, no mapping, segment without a source,sourceRoot, string/Bufferinput, a mapOriginalSourceproduced) andtest/package-entry.js. 100% line and branch coverage of the new file.Does this PR introduce a breaking change?
No.
If relevant, what needs to be documented once your changes are merged or what have you already documented?
Documented in
README.md(util.sourceMap).Use of AI
Written with Claude Code from my requirements; I reviewed the API and the tests.
Generated by Claude Code
Summary by CodeRabbit
util.sourceMap.originalPositionForto look up the original source position for a generated line and column. It accepts a source map object, JSON string, or buffer, and returns the closest mapping at or before the requested column—ornullif none exists.sourceRoothandling, and whole-map decoding behavior for one-off lookups.