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[Flight] Warn once if eval is disabled in dev environment #35661

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Merged
eps1lon merged 2 commits into from
Feb 2, 2026
Merged

[Flight] Warn once if eval is disabled in dev environment #35661

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[Flight] Warn once if `eval` is disabled in dev environment
eps1lon Jan 30, 2026
d2430b3
Move to reply client
eps1lon Feb 2, 2026
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36 changes: 36 additions & 0 deletions packages/react-client/src/ReactClientDebugConfigBrowser.js
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Original file line number Diff line number Diff line change
@@ -0,0 +1,36 @@
/**
* Copyright (c) Meta Platforms, Inc. and affiliates.
*
* This source code is licensed under the MIT license found in the
* LICENSE file in the root directory of this source tree.
*
* @flow
*/

let hasConfirmedEval = false;
export function checkEvalAvailabilityOnceDev(): void {
if (__DEV__) {
if (!hasConfirmedEval) {
hasConfirmedEval = true;
try {
// eslint-disable-next-line no-eval
(0, eval)('null');
} 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',
);
}
Comment on lines +18 to +27
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@sohammeta sohammeta Jan 31, 2026

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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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@eps1lon eps1lon Feb 2, 2026

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What is the thrown error saying, that's not included in the message we post?

}
} else {
// These errors should never make it into a build so we don't need to encode them in codes.json
// eslint-disable-next-line react-internal/prod-error-codes
throw new Error(
'checkEvalAvailabilityOnceDev should never be called in production mode. This is a bug in React.',
);
}
}
36 changes: 36 additions & 0 deletions packages/react-client/src/ReactClientDebugConfigNode.js
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Original file line number Diff line number Diff line change
@@ -0,0 +1,36 @@
/**
* Copyright (c) Meta Platforms, Inc. and affiliates.
*
* This source code is licensed under the MIT license found in the
* LICENSE file in the root directory of this source tree.
*
* @flow
*/

let hasConfirmedEval = false;
export function checkEvalAvailabilityOnceDev(): void {
if (__DEV__) {
if (!hasConfirmedEval) {
hasConfirmedEval = true;
try {
// eslint-disable-next-line no-eval
(0, eval)('null');
} catch {
console.error(
'eval() is not supported in this environment. ' +
'This can happen if you started the Node.js process with --disallow-code-generation-from-strings, ' +
'or if `eval` was patched by other means. ' +
'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',
);
}
}
} else {
// These errors should never make it into a build so we don't need to encode them in codes.json
// eslint-disable-next-line react-internal/prod-error-codes
throw new Error(
'checkEvalAvailabilityOnceDev should never be called in production mode. This is a bug in React.',
);
}
}
34 changes: 34 additions & 0 deletions packages/react-client/src/ReactClientDebugConfigPlain.js
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Original file line number Diff line number Diff line change
@@ -0,0 +1,34 @@
/**
* Copyright (c) Meta Platforms, Inc. and affiliates.
*
* This source code is licensed under the MIT license found in the
* LICENSE file in the root directory of this source tree.
*
* @flow
*/

let hasConfirmedEval = false;
export function checkEvalAvailabilityOnceDev(): void {
if (__DEV__) {
if (!hasConfirmedEval) {
hasConfirmedEval = true;
try {
// eslint-disable-next-line no-eval
(0, eval)('null');
} catch {
console.error(
'eval() is not supported in this environment. ' +
'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',
);
}
}
} else {
// These errors should never make it into a build so we don't need to encode them in codes.json
// eslint-disable-next-line react-internal/prod-error-codes
throw new Error(
'checkEvalAvailabilityOnceDev should never be called in production mode. This is a bug in React.',
);
}
}
9 changes: 9 additions & 0 deletions packages/react-client/src/ReactFlightClient.js
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Original file line number Diff line number Diff line change
Expand Up @@ -61,6 +61,7 @@ import {
bindToConsole,
rendererVersion,
rendererPackageName,
checkEvalAvailabilityOnceDev,
} from './ReactFlightClientConfig';

import {
Expand Down Expand Up @@ -2768,6 +2769,14 @@ export function createResponse(
debugEndTime: void | number, // DEV-only
debugChannel: void | DebugChannel, // DEV-only
): WeakResponse {
if (__DEV__) {
// We use eval to create fake function stacks which includes Component stacks.
// 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();
}

return getWeakResponse(
// $FlowFixMe[invalid-constructor]: the shapes are exact here but Flow doesn't like constructors
new ResponseInstance(
Expand Down
13 changes: 12 additions & 1 deletion packages/react-client/src/ReactFlightReplyClient.js
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Expand Up @@ -39,7 +39,10 @@ import getPrototypeOf from 'shared/getPrototypeOf';

const ObjectPrototype = Object.prototype;

import {usedWithSSR} from './ReactFlightClientConfig';
import {
usedWithSSR,
checkEvalAvailabilityOnceDev,
} from './ReactFlightClientConfig';

type ReactJSONValue =
| string
Expand Down Expand Up @@ -190,6 +193,14 @@ export function processReply(
const writtenObjects: WeakMap<Reference, string> = new WeakMap();
let modelRoot: null | ReactServerValue = root;

if (__DEV__) {
// We use eval to create fake function stacks which includes Component stacks.
// 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();
}

function serializeTypedArray(
tag: string,
typedArray: $ArrayBufferView,
Expand Down
6 changes: 6 additions & 0 deletions packages/react-client/src/__tests__/ReactFlight-test.js
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Expand Up @@ -3715,6 +3715,12 @@ describe('ReactFlight', () => {
'\n in b (at **)' +
'\n in a (at **)',
);
assertConsoleErrorDev([
'eval() is not supported in this environment. ' +
'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',
]);
} else {
expect(receivedError.message).toEqual(
'An error occurred in the Server Components render. The specific message is omitted in production builds to avoid leaking sensitive details. A digest property is included on this error instance which may provide additional details about the nature of the error.',
Expand Down
3 changes: 3 additions & 0 deletions packages/react-client/src/forks/ReactFlightClientConfig.custom.js
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Expand Up @@ -53,3 +53,6 @@ export const bindToConsole = $$$config.bindToConsole;

export const rendererVersion = $$$config.rendererVersion;
export const rendererPackageName = $$$config.rendererPackageName;

export const checkEvalAvailabilityOnceDev =
$$$config.checkEvalAvailabilityOnceDev;
1 change: 1 addition & 0 deletions packages/react-client/src/forks/ReactFlightClientConfig.dom-browser-esm.js
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Expand Up @@ -12,6 +12,7 @@ export const rendererPackageName = 'react-server-dom-esm';

export * from 'react-client/src/ReactFlightClientStreamConfigWeb';
export * from 'react-client/src/ReactClientConsoleConfigBrowser';
export * from 'react-client/src/ReactClientDebugConfigBrowser';
export * from 'react-server-dom-esm/src/client/ReactFlightClientConfigBundlerESM';
export * from 'react-server-dom-esm/src/client/ReactFlightClientConfigTargetESMBrowser';
export * from 'react-dom-bindings/src/shared/ReactFlightClientConfigDOM';
Expand Down
1 change: 1 addition & 0 deletions packages/react-client/src/forks/ReactFlightClientConfig.dom-browser-parcel.js
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Expand Up @@ -12,6 +12,7 @@ export const rendererPackageName = 'react-server-dom-parcel';

export * from 'react-client/src/ReactFlightClientStreamConfigWeb';
export * from 'react-client/src/ReactClientConsoleConfigBrowser';
export * from 'react-client/src/ReactClientDebugConfigBrowser';
export * from 'react-server-dom-parcel/src/client/ReactFlightClientConfigBundlerParcel';
export * from 'react-server-dom-parcel/src/client/ReactFlightClientConfigTargetParcelBrowser';
export * from 'react-dom-bindings/src/shared/ReactFlightClientConfigDOM';
Expand Down
1 change: 1 addition & 0 deletions packages/react-client/src/forks/ReactFlightClientConfig.dom-browser-turbopack.js
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Expand Up @@ -12,6 +12,7 @@ export const rendererPackageName = 'react-server-dom-turbopack';

export * from 'react-client/src/ReactFlightClientStreamConfigWeb';
export * from 'react-client/src/ReactClientConsoleConfigBrowser';
export * from 'react-client/src/ReactClientDebugConfigBrowser';
export * from 'react-server-dom-turbopack/src/client/ReactFlightClientConfigBundlerTurbopack';
export * from 'react-server-dom-turbopack/src/client/ReactFlightClientConfigBundlerTurbopackBrowser';
export * from 'react-server-dom-turbopack/src/client/ReactFlightClientConfigTargetTurbopackBrowser';
Expand Down
1 change: 1 addition & 0 deletions packages/react-client/src/forks/ReactFlightClientConfig.dom-browser.js
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Expand Up @@ -12,6 +12,7 @@ export const rendererPackageName = 'react-server-dom-webpack';

export * from 'react-client/src/ReactFlightClientStreamConfigWeb';
export * from 'react-client/src/ReactClientConsoleConfigBrowser';
export * from 'react-client/src/ReactClientDebugConfigBrowser';
export * from 'react-server-dom-webpack/src/client/ReactFlightClientConfigBundlerWebpack';
export * from 'react-server-dom-webpack/src/client/ReactFlightClientConfigBundlerWebpackBrowser';
export * from 'react-server-dom-webpack/src/client/ReactFlightClientConfigTargetWebpackBrowser';
Expand Down
1 change: 1 addition & 0 deletions packages/react-client/src/forks/ReactFlightClientConfig.dom-bun.js
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Expand Up @@ -12,6 +12,7 @@ export const rendererPackageName = 'react-server-dom-bun';

export * from 'react-client/src/ReactFlightClientStreamConfigWeb';
export * from 'react-client/src/ReactClientConsoleConfigPlain';
export * from 'react-client/src/ReactClientDebugConfigPlain';
export * from 'react-dom-bindings/src/shared/ReactFlightClientConfigDOM';

export opaque type ModuleLoading = mixed;
Expand Down
1 change: 1 addition & 0 deletions packages/react-client/src/forks/ReactFlightClientConfig.dom-edge-parcel.js
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Expand Up @@ -12,6 +12,7 @@ export const rendererPackageName = 'react-server-dom-parcel';

export * from 'react-client/src/ReactFlightClientStreamConfigWeb';
export * from 'react-client/src/ReactClientConsoleConfigServer';
export * from 'react-client/src/ReactClientDebugConfigPlain';
export * from 'react-server-dom-parcel/src/client/ReactFlightClientConfigBundlerParcel';
export * from 'react-server-dom-parcel/src/client/ReactFlightClientConfigTargetParcelServer';
export * from 'react-dom-bindings/src/shared/ReactFlightClientConfigDOM';
Expand Down
1 change: 1 addition & 0 deletions packages/react-client/src/forks/ReactFlightClientConfig.dom-edge-turbopack.js
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Expand Up @@ -12,6 +12,7 @@ export const rendererPackageName = 'react-server-dom-turbopack';

export * from 'react-client/src/ReactFlightClientStreamConfigWeb';
export * from 'react-client/src/ReactClientConsoleConfigServer';
export * from 'react-client/src/ReactClientDebugConfigPlain';
export * from 'react-server-dom-turbopack/src/client/ReactFlightClientConfigBundlerTurbopack';
export * from 'react-server-dom-turbopack/src/client/ReactFlightClientConfigBundlerTurbopackServer';
export * from 'react-server-dom-turbopack/src/client/ReactFlightClientConfigTargetTurbopackServer';
Expand Down
1 change: 1 addition & 0 deletions packages/react-client/src/forks/ReactFlightClientConfig.dom-edge-webpack.js
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Expand Up @@ -12,6 +12,7 @@ export const rendererPackageName = 'react-server-dom-webpack';

export * from 'react-client/src/ReactFlightClientStreamConfigWeb';
export * from 'react-client/src/ReactClientConsoleConfigServer';
export * from 'react-client/src/ReactClientDebugConfigPlain';
export * from 'react-server-dom-webpack/src/client/ReactFlightClientConfigBundlerWebpack';
export * from 'react-server-dom-webpack/src/client/ReactFlightClientConfigBundlerWebpackServer';
export * from 'react-server-dom-webpack/src/client/ReactFlightClientConfigTargetWebpackServer';
Expand Down
1 change: 1 addition & 0 deletions packages/react-client/src/forks/ReactFlightClientConfig.dom-legacy.js
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Expand Up @@ -12,6 +12,7 @@ export const rendererPackageName = 'not-used';

export * from 'react-client/src/ReactFlightClientStreamConfigWeb';
export * from 'react-client/src/ReactClientConsoleConfigBrowser';
export * from 'react-client/src/ReactClientDebugConfigBrowser';

export type Response = any;
export opaque type ModuleLoading = mixed;
Expand Down
1 change: 1 addition & 0 deletions packages/react-client/src/forks/ReactFlightClientConfig.dom-node-esm.js
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Expand Up @@ -12,6 +12,7 @@ export const rendererPackageName = 'react-server-dom-esm';

export * from 'react-client/src/ReactFlightClientStreamConfigNode';
export * from 'react-client/src/ReactClientConsoleConfigServer';
export * from 'react-client/src/ReactClientDebugConfigNode';
export * from 'react-server-dom-esm/src/client/ReactFlightClientConfigBundlerESM';
export * from 'react-server-dom-esm/src/client/ReactFlightClientConfigTargetESMServer';
export * from 'react-dom-bindings/src/shared/ReactFlightClientConfigDOM';
Expand Down
1 change: 1 addition & 0 deletions packages/react-client/src/forks/ReactFlightClientConfig.dom-node-parcel.js
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Expand Up @@ -12,6 +12,7 @@ export const rendererPackageName = 'react-server-dom-parcel';

export * from 'react-client/src/ReactFlightClientStreamConfigNode';
export * from 'react-client/src/ReactClientConsoleConfigServer';
export * from 'react-client/src/ReactClientDebugConfigNode';
export * from 'react-server-dom-parcel/src/client/ReactFlightClientConfigBundlerParcel';
export * from 'react-server-dom-parcel/src/client/ReactFlightClientConfigTargetParcelServer';
export * from 'react-dom-bindings/src/shared/ReactFlightClientConfigDOM';
Expand Down
1 change: 1 addition & 0 deletions packages/react-client/src/forks/ReactFlightClientConfig.dom-node-turbopack.js
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Expand Up @@ -12,6 +12,7 @@ export const rendererPackageName = 'react-server-dom-turbopack';

export * from 'react-client/src/ReactFlightClientStreamConfigNode';
export * from 'react-client/src/ReactClientConsoleConfigServer';
export * from 'react-client/src/ReactClientDebugConfigNode';
export * from 'react-server-dom-turbopack/src/client/ReactFlightClientConfigBundlerTurbopack';
export * from 'react-server-dom-turbopack/src/client/ReactFlightClientConfigBundlerTurbopackServer';
export * from 'react-server-dom-turbopack/src/client/ReactFlightClientConfigTargetTurbopackServer';
Expand Down
1 change: 1 addition & 0 deletions packages/react-client/src/forks/ReactFlightClientConfig.dom-node-unbundled.js
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Expand Up @@ -12,6 +12,7 @@ export const rendererPackageName = 'react-server-dom-unbundled';

export * from 'react-client/src/ReactFlightClientStreamConfigNode';
export * from 'react-client/src/ReactClientConsoleConfigServer';
export * from 'react-client/src/ReactClientDebugConfigNode';
export * from 'react-server-dom-unbundled/src/client/ReactFlightClientConfigBundlerNode';
export * from 'react-server-dom-unbundled/src/client/ReactFlightClientConfigTargetNodeServer';
export * from 'react-dom-bindings/src/shared/ReactFlightClientConfigDOM';
Expand Down
1 change: 1 addition & 0 deletions packages/react-client/src/forks/ReactFlightClientConfig.dom-node.js
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Expand Up @@ -11,6 +11,7 @@ export const rendererPackageName = 'react-server-dom-webpack';

export * from 'react-client/src/ReactFlightClientStreamConfigNode';
export * from 'react-client/src/ReactClientConsoleConfigServer';
export * from 'react-client/src/ReactClientDebugConfigNode';
export * from 'react-server-dom-webpack/src/client/ReactFlightClientConfigBundlerWebpack';
export * from 'react-server-dom-webpack/src/client/ReactFlightClientConfigBundlerWebpackServer';
export * from 'react-server-dom-webpack/src/client/ReactFlightClientConfigTargetWebpackServer';
Expand Down
1 change: 1 addition & 0 deletions packages/react-client/src/forks/ReactFlightClientConfig.markup.js
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Expand Up @@ -14,6 +14,7 @@ import type {Thenable} from 'shared/ReactTypes';

export * from 'react-markup/src/ReactMarkupLegacyClientStreamConfig.js';
export * from 'react-client/src/ReactClientConsoleConfigPlain';
export * from 'react-client/src/ReactClientDebugConfigPlain';

export type ModuleLoading = null;
export type ServerConsumerModuleMap = null;
Expand Down
25 changes: 25 additions & 0 deletions packages/react-noop-renderer/src/ReactNoopFlightClient.js
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Expand Up @@ -53,6 +53,7 @@ const {createResponse, createStreamState, processBinaryChunk, getRoot, close} =
[console].concat(args),
);
},
checkEvalAvailabilityOnceDev,
});

type ReadOptions = {|
Expand Down Expand Up @@ -87,4 +88,28 @@ function read<T>(source: Source, options: ReadOptions): Thenable<T> {
return getRoot(response);
}

let hasConfirmedEval = false;
function checkEvalAvailabilityOnceDev(): void {
if (__DEV__) {
if (!hasConfirmedEval) {
hasConfirmedEval = true;
try {
// eslint-disable-next-line no-eval
(0, eval)('null');
} catch {
console.error(
'eval() is not supported in this environment. ' +
'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',
);
}
}
} else {
throw new Error(
'checkEvalAvailabilityOnceDev should never be called in production mode. This is a bug in React.',
);
}
}

export {read};
32 changes: 32 additions & 0 deletions packages/react-server-dom-turbopack/src/__tests__/ReactFlightTurbopackDOMReply-test.js
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Expand Up @@ -18,6 +18,7 @@ global.TextEncoder = require('util').TextEncoder;
global.TextDecoder = require('util').TextDecoder;

// let serverExports;
let assertConsoleErrorDev;
let turbopackServerMap;
let ReactServerDOMServer;
let ReactServerDOMClient;
Expand All @@ -41,6 +42,9 @@ describe('ReactFlightTurbopackDOMReply', () => {
ReactServerDOMServer = require('react-server-dom-turbopack/server.browser');
jest.resetModules();
ReactServerDOMClient = require('react-server-dom-turbopack/client');

const InternalTestUtils = require('internal-test-utils');
assertConsoleErrorDev = InternalTestUtils.assertConsoleErrorDev;
});

it('can encode a reply', async () => {
Expand All @@ -52,4 +56,32 @@ describe('ReactFlightTurbopackDOMReply', () => {

expect(decoded).toEqual({some: 'object'});
});

it('warns with a tailored message if eval is not available in dev', async () => {
// eslint-disable-next-line no-eval
const previousEval = globalThis.eval.bind(globalThis);
// eslint-disable-next-line no-eval
globalThis.eval = () => {
throw new Error('eval is disabled');
};

try {
const body = await ReactServerDOMClient.encodeReply({some: 'object'});
assertConsoleErrorDev([
'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',
]);

await ReactServerDOMServer.decodeReply(body, turbopackServerMap);

assertConsoleErrorDev([]);
} finally {
// eslint-disable-next-line no-eval
globalThis.eval = previousEval;
}
});
});
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