facebook/react · warning

More than one external source map detected in the source fil

Error message

More than one external source map detected in the source file; skipping "${sourceMappingURL}"

What it means

While parsing hook names, loadSourceAndMetadata (packages/react-devtools-shared/src/hooks/parseHookNames/loadSourceAndMetadata.js) scans runtime source code with a sourceMappingURL regex to find external source maps. Well-formed files have exactly one match; if the regex finds several (usually a string literal in the source that looks like a source-map comment, or a concatenated bundle), DevTools cannot tell which is real, so it skips all but the last one and warns per skipped URL. Hook-name resolution then proceeds with the surviving (last) map.

Source

Thrown at packages/react-devtools-shared/src/hooks/parseHookNames/loadSourceAndMetadata.js:239

        } else {
          externalSourceMapURLs.push(sourceMappingURL);
        }

        // If the first source map we found wasn't a match, check for more.
        sourceMappingURLMatch = withSyncPerfMeasurements(
          'sourceMapRegex.exec(runtimeSourceCode)',
          () => sourceMapRegex.exec(runtimeSourceCode),
        );
      }

      if (hookSourceAndMetadata.sourceMapJSON === null) {
        externalSourceMapURLs.forEach((sourceMappingURL, index) => {
          if (index !== externalSourceMapURLs.length - 1) {
            // Files with external source maps should only have a single source map.
            // More than one result might indicate an edge case,
            // like a string in the source code that matched our "sourceMappingURL" regex.
            // We should just skip over cases like this.
            console.warn(
              `More than one external source map detected in the source file; skipping "${sourceMappingURL}"`,
            );
            return;
          }

          const {runtimeSourceURL} = hookSourceAndMetadata;
          let url = sourceMappingURL;
          if (!url.startsWith('http') && !url.startsWith('/')) {
            // Resolve paths relative to the location of the file name
            const lastSlashIdx = runtimeSourceURL.lastIndexOf('/');
            if (lastSlashIdx !== -1) {
              const baseURL = runtimeSourceURL.slice(
                0,
                runtimeSourceURL.lastIndexOf('/'),
              );
              url = `${baseURL}/${url}`;
            }
          }

View on GitHub (pinned to eafeac097b)

Solutions

1. Verify the final bundle has a single //# sourceMappingURL comment at the very end (standard bundler output does).
2. If your source contains literal strings that match the pattern, ignore the warning — DevTools uses the last match, which is normally the real one.
3. Rebuild so source maps are inline or emitted once (avoid concatenating multiple already-mapped files without a final sourcemap merge).

// Before enabling 'parse hook names', sanity-check the bundle:
const matches = bundleSource.match(/^\/\/# sourceMappingURL=.*/gm) ?? [];
if (matches.length > 1) {
  // extra matches are likely string literals; the last one is used by DevTools
  console.log('multiple sourceMappingURL candidates:', matches);
}

When it happens

Trigger: The inspected bundle contains multiple strings matching the sourceMappingURL pattern — e.g. bundler/bootloader source code that embeds such strings, concatenated files, or code that documents source maps in string literals — and you use the 'parse hook names' feature in the Components panel.

Common situations: Debugging bundler-adjacent code, devtools-in-devtools setups, or libraries whose source legitimately contains 'sourceMappingURL' text; bundles produced by naive concatenation of individually mapped files.

Related errors

• Cannot not suspend ID '${suspendedSet[i]}'.
• Could not find instance with id "${id}"
• Could not find element with id "${id}"
• Invalid renderer id "${rendererID}" for element "${id}"
• Invalid renderer id "${match.rendererID}" for element "${mat

AI-assisted analysis of facebook/react@eafeac097b (2026-08-21). Data as JSON: /api/errors/7fede6311f541db6. Report an issue: GitHub.