From f247ebaf44317ac6648b62f99ceaed1e4fc4dc01 Mon Sep 17 00:00:00 2001 From: Tim Neutkens Date: Thu, 19 Feb 2026 17:37:41 +0100 Subject: [PATCH] [Flight] Walk parsed JSON instead of using reviver for parsing RSC payload (#35776) MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit ## Summary Follow-up to https://github.com/vercel/next.js/pull/89823 with the actual changes to React. Replaces the `JSON.parse` reviver callback in `initializeModelChunk` with a two-step approach: plain `JSON.parse()` followed by a recursive `reviveModel()` post-process (same as in Flight Reply Server). This yields a **~75% speedup** in RSC chunk deserialization. | Payload | Original (ms) | Walk (ms) | Speedup | |---------|---------------|-----------|---------| | Small (2 elements, 142B) | 0.0024 | 0.0007 | **+72%** | | Medium (~12 elements, 914B) | 0.0116 | 0.0031 | **+73%** | | Large (~90 elements, 16.7KB) | 0.1836 | 0.0451 | **+75%** | | XL (~200 elements, 25.7KB) | 0.3742 | 0.0913 | **+76%** | | Table (1000 rows, 110KB) | 3.0862 | 0.6887 | **+78%** | ## Problem `createFromJSONCallback` returns a reviver function passed as the second argument to `JSON.parse()`. This reviver is called for **every key-value pair** in the parsed JSON. While the logic inside the reviver is lightweight, the dominant cost is the **C++ → JavaScript boundary crossing** — V8's `JSON.parse` is implemented in C++, and calling back into JavaScript for every node incurs significant overhead. Even a trivial no-op reviver `(k, v) => v` makes `JSON.parse` **~4x slower** than bare `JSON.parse` without a reviver: ``` 108 KB payload: Bare JSON.parse: 0.60 ms Trivial reviver: 2.95 ms (+391%) ``` ## Change Replace the reviver with a two-step process: 1. `JSON.parse(resolvedModel)` — parse the entire payload in C++ with no callbacks 2. `reviveModel` — recursively walk the resulting object in pure JavaScript to apply RSC transformations The `reviveModel` function includes additional optimizations over the original reviver: - **Short-circuits plain strings**: only calls `parseModelString` when the string starts with `$`, skipping the vast majority of strings (class names, text content, etc.) - **Stays entirely in JavaScript** — no C++ boundary crossings during the walk ## Results You can find the related applications in the [Next.js PR ](https://github.com/vercel/next.js/pull/89823)as I've been testing this on Next.js applications. ### Table as Server Component with 1000 items Before: ```     "min": 13.782875000000786,     "max": 22.23400000000038,     "avg": 17.116868530000083,     "p50": 17.10766700000022,     "p75": 18.50787499999933,     "p95": 20.426249999998618,     "p99": 21.814125000000786 ``` After: ```     "min": 10.963916999999128,     "max": 18.096083000000363,     "avg": 13.543286884999988,     "p50": 13.58350000000064,     "p75": 14.871791999999914,     "p95": 16.08429099999921,     "p99": 17.591458000000785 ``` ### Table as Client Component with 1000 items Before: ```     "min": 3.888875000000553,     "max": 9.044959000000745,     "avg": 4.651271475000067,     "p50": 4.555749999999534,     "p75": 4.966624999999112,     "p95": 5.47754200000054,     "p99": 6.109499999998661 ```` After: ```     "min": 3.5986250000005384,     "max": 5.374291000000085,     "avg": 4.142990245000046,     "p50": 4.10570799999914,     "p75": 4.392041999999492,     "p95": 4.740084000000934,     "p99": 5.1652500000000146 ``` ### Nested Suspense Before: ``` Requests: 200 Min: 73ms Max: 106ms Avg: 78ms P50: 77ms P75: 80ms P95: 85ms P99: 94ms ``` After: ``` Requests: 200 Min: 56ms Max: 67ms Avg: 59ms P50: 58ms P75: 60ms P95: 65ms P99: 66ms ``` ### Even more nested Suspense (double-level Suspense) Before: ``` Requests: 200 Min: 159ms Max: 208ms Avg: 169ms P50: 167ms P75: 173ms P95: 183ms P99: 188ms ``` After: ``` Requests: 200 Min: 125ms Max: 170ms Avg: 134ms P50: 132ms P75: 138ms P95: 148ms P99: 160ms ``` ## How did you test this change? Ran it across many Next.js benchmark applications. The entire Next.js test suite passes with this change. --------- Co-authored-by: Hendrik Liebau --- .../react-client/src/ReactFlightClient.js | 65 ++++++++++++++----- .../src/ReactNoopFlightClient.js | 3 - 2 files changed, 49 insertions(+), 19 deletions(-) diff --git a/packages/react-client/src/ReactFlightClient.js b/packages/react-client/src/ReactFlightClient.js index 48a8c2740f49..fbf5190ed516 100644 --- a/packages/react-client/src/ReactFlightClient.js +++ b/packages/react-client/src/ReactFlightClient.js @@ -355,7 +355,6 @@ type Response = { _encodeFormAction: void | EncodeFormActionCallback, _nonce: ?string, _chunks: Map>, - _fromJSON: (key: string, value: JSONValue) => any, _stringDecoder: StringDecoder, _closed: boolean, _closedReason: mixed, @@ -2302,6 +2301,11 @@ function defineLazyGetter( // TODO: We should ideally throw here to indicate a difference. return OMITTED_PROP_ERROR; }, + // no-op: the walk function may try to reassign this property after + // parseModelString returns. With the JSON.parse reviver, the engine's + // internal CreateDataProperty silently failed. We use a no-op setter + // to match that behavior in strict mode. + set: function () {}, enumerable: true, configurable: false, }); @@ -2622,6 +2626,11 @@ function parseModelString( // TODO: We should ideally throw here to indicate a difference. return OMITTED_PROP_ERROR; }, + // no-op: the walk function may try to reassign this property + // after parseModelString returns. With the JSON.parse reviver, + // the engine's internal CreateDataProperty silently failed. + // We use a no-op setter to match that behavior in strict mode. + set: function () {}, enumerable: true, configurable: false, }); @@ -2699,7 +2708,6 @@ function ResponseInstance( this._nonce = nonce; this._chunks = chunks; this._stringDecoder = createStringDecoder(); - this._fromJSON = (null: any); this._closed = false; this._closedReason = null; this._allowPartialStream = allowPartialStream; @@ -2783,9 +2791,6 @@ function ResponseInstance( markAllTracksInOrder(); } } - - // Don't inline this call because it causes closure to outline the call above. - this._fromJSON = createFromJSONCallback(this); } export function createResponse( @@ -5259,24 +5264,52 @@ export function processStringChunk( } function parseModel(response: Response, json: UninitializedModel): T { - return JSON.parse(json, response._fromJSON); + const rawModel = JSON.parse(json); + // Pass a wrapper object as parentObject to match the original JSON.parse + // reviver behavior, where the root value's reviver receives {"": rootValue} + // as `this`. This ensures parentObject is never null when accessed downstream. + return reviveModel(response, rawModel, {'': rawModel}, ''); } -function createFromJSONCallback(response: Response) { - // $FlowFixMe[missing-this-annot] - return function (key: string, value: JSONValue) { - if (key === __PROTO__) { - return undefined; +function reviveModel( + response: Response, + value: JSONValue, + parentObject: Object, + key: string, +): any { + if (typeof value === 'string') { + if (value[0] === '$') { + return parseModelString(response, parentObject, key, value); } - if (typeof value === 'string') { - // We can't use .bind here because we need the "this" value. - return parseModelString(response, this, key, value); + return value; + } + if (typeof value !== 'object' || value === null) { + return value; + } + if (isArray(value)) { + for (let i = 0; i < value.length; i++) { + (value: any)[i] = reviveModel(response, value[i], value, '' + i); } - if (typeof value === 'object' && value !== null) { + if (value[0] === REACT_ELEMENT_TYPE) { + // React element tuple return parseModelTuple(response, value); } return value; - }; + } + // Plain object + for (const k in value) { + if (k === __PROTO__) { + delete (value: any)[k]; + } else { + const walked = reviveModel(response, (value: any)[k], value, k); + if (walked !== undefined) { + (value: any)[k] = walked; + } else { + delete (value: any)[k]; + } + } + } + return value; } export function close(weakResponse: WeakResponse): void { diff --git a/packages/react-noop-renderer/src/ReactNoopFlightClient.js b/packages/react-noop-renderer/src/ReactNoopFlightClient.js index a5c43bd65259..4699b149e85f 100644 --- a/packages/react-noop-renderer/src/ReactNoopFlightClient.js +++ b/packages/react-noop-renderer/src/ReactNoopFlightClient.js @@ -43,9 +43,6 @@ const {createResponse, createStreamState, processBinaryChunk, getRoot, close} = requireModule(idx: string) { return readModule(idx); }, - parseModel(response: Response, json) { - return JSON.parse(json, response._fromJSON); - }, bindToConsole(methodName, args, badgeName) { return Function.prototype.bind.apply( // eslint-disable-next-line react-internal/no-production-logging