facebook/react · error · Error

The source map has more mappings than there are lines.

Error message

The source map has more mappings than there are lines.

What it means

ReactFlightWebpackNodeLoader instruments 'use client'/'use server' modules and merges the file's existing source map with the mappings it inserts. As a consistency check it compares the number of mapped lines against program.loc.end.line of the actual source; if the source map claims mappings past the last line of the file, the map cannot belong to this source, so the loader refuses to continue.

Source

Thrown at packages/react-server-dom-webpack/src/ReactFlightWebpackNodeLoader.js:364

          exportedEntries[nextEntryIdx].originalColumn = lastOriginalColumn;
          exportedEntries[nextEntryIdx].originalSource = lastSourceIndex;
          exportedEntries[nextEntryIdx].nameIndex = lastNameIndex;
        }
      }

      for (
        let lastIdx = mappings.length - 1;
        lastIdx >= 0 && mappings[lastIdx] === ';';
        lastIdx--
      ) {
        // If the last mapped lines don't contain any segments, we don't get a callback from readMappings
        // so we need to pad the number of mapped lines, with one for each empty line.
        lastMappedLine++;
      }

      sourceLineCount = program.loc.end.line;
      if (sourceLineCount < lastMappedLine) {
        throw new Error(
          'The source map has more mappings than there are lines.',
        );
      }
      // If the original source string had more lines than there are mappings in the source map.
      // Add some extra padding of unmapped lines so that any lines that we add line up.
      for (
        let extraLines = sourceLineCount - lastMappedLine;
        extraLines > 0;
        extraLines--
      ) {
        mappings += ';';
      }
    } else {
      // If a file doesn't have a source map then we generate a blank source map that just
      // contains the original content and segments pointing to the original lines.
      sourceLineCount = 1;
      let idx = -1;
      while ((idx = source.indexOf('\n', idx + 1)) !== -1) {

View on GitHub (pinned to eafeac097b)

Solutions

1. Regenerate source maps for the file so they match the current source exactly
2. Reorder/repair the transform chain so each stage's map corresponds to its input (source-map chaining)
3. If the map is expendable, remove the sourceMappingURL comment for that file so the loader skips merging
4. Pin consistent tool versions across the chain so map semantics do not drift

Example fix

# before: stale map next to a regenerated source
src/Component.js      (edited, now 120 lines)
src/Component.js.map  (maps 130 lines)

# after: rebuild maps in the same pass as sources
npx babel src --out-dir lib --source-maps
# or drop the stale map
rm src/Component.js.map  # loader then skips source-map merging

// Pre-validate maps before the loader sees them
import fs from 'fs';
function mapIsValid(jsPath, mapPath) {
  const map = JSON.parse(fs.readFileSync(mapPath, 'utf8'));
  const mappedLines = map.mappings.split(';').length;
  const srcLines = fs.readFileSync(jsPath, 'utf8').split('\n').length;
  return mappedLines <= srcLines;
}

When it happens

Trigger: A processed file carries a sourceMappingURL whose map has more mapping lines than the file has lines — typically a map generated from a different (longer) version of the file, a misaligned transform step, or a truncated/malformed mappings field.

Common situations: Chained Babel/TS/SWC transforms where a later stage emits a map for the pre-transform source | Stale .map files next to regenerated sources | Tools that append or strip lines (shebang removal, banner plugins) without updating the map

Related errors

• The source map has more mappings than there are lines.
• The source map has more mappings than there are lines.
• Could not find the module "${modulePath}" in the React Clien
• The module "${modulePath}" is marked as an async ESM module
• Use react-server-dom-webpack/client instead.

AI-assisted analysis of facebook/react@eafeac097b (2026-08-21). Data as JSON: /api/errors/38ff5d29b5c3cc51. Report an issue: GitHub.