[Flight] Warn once if eval is disabled in dev environment
#35661
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Jan 30, 2026
| // A warning would be noise if you used Flight without Components and don't encounter | ||
| // errors. We're warning eagerly so that you configure your environment accordingly | ||
| // before you encounter an error. | ||
| checkEvalAvailabilityOnceDev(); |
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Shouldn't this be in ReactFlightReplyClient.js where eval is used? Looks like ReactFlightReplyServer.js does not use eval.
sohammeta
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Jan 31, 2026
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| } catch { | ||
| console.error( | ||
| 'eval() is not supported in this environment. ' + | ||
| 'If this page was served with a `Content-Security-Policy` header, ' + | ||
| 'make sure that `unsafe-eval` is included. ' + | ||
| 'React requires eval() in development mode for various debugging features ' + | ||
| 'like reconstructing callstacks from a different environment.\n' + | ||
| 'React will never use eval() in production mode', | ||
| ); | ||
| } |
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If eval throws for reasons other than being unavailable, those errors will be swallowed.
Suggested change
| } catch { | |
| console.error( | |
| 'eval() is not supported in this environment. ' + | |
| 'If this page was served with a `Content-Security-Policy` header, ' + | |
| 'make sure that `unsafe-eval` is included. ' + | |
| 'React requires eval() in development mode for various debugging features ' + | |
| 'like reconstructing callstacks from a different environment.\n' + | |
| 'React will never use eval() in production mode', | |
| ); | |
| } | |
| } catch (error) { | |
| console.error( | |
| 'eval() is not supported in this environment. ' + | |
| 'If this page was served with a `Content-Security-Policy` header, ' + | |
| 'make sure that `unsafe-eval` is included. ' + | |
| 'React requires eval() in development mode for various debugging features ' + | |
| 'like reconstructing callstacks from a different environment.\n' + | |
| 'React will never use eval() in production mode', | |
| ,error | |
| ); | |
| } |
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Summary
Follow-up to #35650
React uses
evalin development for Server Components and Server Functions to reconstruct callstacks from different environments.evalcan be a legitimate security concern for production environments. It's oftentimes disabled e.g. in browsers viaContent-Security-Policy.If
evalis disabled in development, those debugging features stop working. Without this change no warning was issued. Now we issue a warning with remedies depending on the environment.For browsers, the CSP header needs to be adjusted. In Node.js,
--disallow-code-generation-from-stringsshould not be used. In other environments (e.g. Bun), we don't have a tailored message since those environments don't have a dedicated API to disableeval.If there are legit concerns about disabling
evalin development this warning could be considered noise and we should revisit.Note that we always warn once you use React Server or React Action APIs even though you may not need to reconstruct a callstack (e.g. no Components used or errors transported). I suspect this to be a rare use cases. Though being prepared for potential errors, isn't the worst idea.
How did you test this change?