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perf: fold codegen literals and inline nested object serialization #868

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234 changes: 203 additions & 31 deletions index.js
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Original file line number Diff line number Diff line change
Expand Up @@ -26,7 +26,23 @@ const {
asUnsafeString
} = serializer

const asInteger = serializer.asInteger.bind(serializer)
const parseInteger = serializer.parseInteger

@Eomm Eomm Aug 15, 2026

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can't we move this declaration with the deconstruction above?


// Inlined here instead of calling the serializer method: a plain function
// call is measurably faster than a bound method call in hot paths.
function asInteger (i) {
if (Number.isInteger(i)) {
return '' + i
} else if (typeof i === 'bigint') {
return i.toString()
}
const integer = parseInteger(i)
// check if number is Infinity or NaN
if (integer === Infinity || integer === -Infinity || integer !== integer) {

@ilteoood ilteoood Aug 27, 2026

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instead of checking for === Infinity, could we use Number.isFinite?

throw new Error('The value "' + i + '" cannot be converted to an integer.')
}
return '' + integer
}

`

Expand Down Expand Up @@ -164,6 +180,7 @@ function build (schema, options) {
recursiveSchemas: new Set(),
recursivePaths: new Set(),
buildingSet: new Set(),
inlinedSchemas: new Set(),
uid: 0
}

Expand Down Expand Up @@ -398,7 +415,7 @@ function buildExtraObjectPropertiesSerializer (context, location, addComma, objV
return code
}

function buildInnerObject (context, location, objVar) {
function buildInnerObject (context, location, objVar, isReturnForm) {
const schema = location.schema

const propertiesLocation = location.getPropertyLocation('properties')
Expand All @@ -422,16 +439,16 @@ function buildInnerObject (context, location, objVar) {
}
}

code += 'json += JSON_STR_BEGIN_OBJECT\n'

const localUid = context.uid++
let addComma = ''
let emptyObjectGuard = null

// propertiesKeys is sorted required-first; the guard checks [0] is required because
// otherwise additionalProperties/patternProperties would emit a stray `{ ,"k":v }`.
if (propertiesKeys.length > 0 && requiredProperties.includes(propertiesKeys[0])) {
// The first property is required, so we don't need a comma.
// For the subsequent properties, we can blindly add a comma.
// The first property is required, so it always emits (or throws): the
// object-open brace is merged into its key prefix. Subsequent properties
// blindly prepend a comma, also merged into their key prefixes.

for (let i = 0; i < propertiesKeys.length; i++) {
const key = propertiesKeys[i]
Expand All @@ -447,22 +464,17 @@ function buildInnerObject (context, location, objVar) {
const defaultValue = resolvedLocation.schema.default
const isRequired = requiredProperties.includes(key)

// i === 0 means it's the first property, and it IS required (due to sort). No comma.
// i > 0 means it follows a required property (or a sequence starting with one). Unconditional comma.
const currentAddComma = i === 0 ? '' : 'json += JSON_STR_COMMA'
const keyPrefix = (i === 0 ? '{' : ',') + sanitizedKey + ':'

code += `
const ${value} = ${objVar}[${sanitizedKey}]
if (${value} !== undefined) {
${currentAddComma}
json += ${JSON.stringify(sanitizedKey + ':')}
${buildValue(context, resolvedLocation, `${value}`)}
${buildValueWithPrefix(context, resolvedLocation, `${value}`, JSON.stringify(keyPrefix), propertyLocation.schema.$ref === undefined)}
}`

if (defaultValue !== undefined) {
code += ` else {
${currentAddComma}
json += ${JSON.stringify(sanitizedKey + ':' + JSON.stringify(defaultValue))}
json += ${JSON.stringify(keyPrefix + JSON.stringify(defaultValue))}
}
`
} else if (isRequired) {
Expand All @@ -477,16 +489,23 @@ function buildInnerObject (context, location, objVar) {

addComma = 'json += JSON_STR_COMMA'
} else {
// No property is guaranteed to be emitted, so the object-open brace is
// merged into whichever property emits first (via the comma flag when
// more than one might emit) and only added standalone when none does.
const needsRuntimeComma = propertiesKeys.length > 1 || schema.patternProperties || (schema.additionalProperties !== undefined && schema.additionalProperties !== false)

if (needsRuntimeComma) {
code += `let addComma_${localUid} = false\n`
addComma = `!addComma_${localUid} && (addComma_${localUid} = true) || (json += JSON_STR_COMMA)`
addComma = `json += addComma_${localUid} ? JSON_STR_COMMA : ((addComma_${localUid} = true), JSON_STR_BEGIN_OBJECT)`
emptyObjectGuard = `if (!addComma_${localUid}) json += JSON_STR_BEGIN_OBJECT\n`
} else if (propertiesKeys.length === 0) {
code += 'json += JSON_STR_BEGIN_OBJECT\n'
}

for (const key of propertiesKeys) {
let propertyLocation = propertiesLocation.getPropertyLocation(key)
if (propertyLocation.schema.$ref) {
const hadRef = propertyLocation.schema.$ref !== undefined
if (hadRef) {
propertyLocation = resolveRef(context, propertyLocation)
}

Expand All @@ -495,18 +514,22 @@ function buildInnerObject (context, location, objVar) {
const defaultValue = propertyLocation.schema.default
const isRequired = requiredProperties.includes(key) // Should be false here but good to keep

const keyColon = sanitizedKey + ':'
const prefixExpr = needsRuntimeComma
? `(addComma_${localUid} ? ${JSON.stringify(',' + keyColon)} : (addComma_${localUid} = true, ${JSON.stringify('{' + keyColon)}))`
: JSON.stringify('{' + keyColon)

code += `
const ${value} = ${objVar}[${sanitizedKey}]
if (${value} !== undefined) {
${addComma}
json += ${JSON.stringify(sanitizedKey + ':')}
${buildValue(context, propertyLocation, `${value}`)}
${buildValueWithPrefix(context, propertyLocation, `${value}`, prefixExpr, !hadRef)}
}`

if (defaultValue !== undefined) {
const defaultLiteral = JSON.stringify(JSON.stringify(defaultValue))
const folded = foldStringConstants(prefixExpr, defaultLiteral)
code += ` else {
${addComma}
json += ${JSON.stringify(sanitizedKey + ':' + JSON.stringify(defaultValue))}
json += ${folded !== null ? folded : `${prefixExpr}\n json += ${defaultLiteral}`}
}
`
} else if (isRequired) {
Expand All @@ -515,6 +538,13 @@ function buildInnerObject (context, location, objVar) {
throw new Error('${sanitizedKey.replace(/'/g, '\\\'')} is required!')
}
`
} else if (!needsRuntimeComma) {
// Single optional property: the object-open brace is merged into its
// key, so it must still be emitted when the property is absent.
code += ` else {
json += JSON_STR_BEGIN_OBJECT
}
`
} else {
code += '\n'
}
Expand All @@ -525,9 +555,13 @@ function buildInnerObject (context, location, objVar) {
code += buildExtraObjectPropertiesSerializer(context, location, addComma, objVar)
}

code += `
json += JSON_STR_END_OBJECT
`
if (emptyObjectGuard !== null) {
code += emptyObjectGuard
}

code += isReturnForm
? '\nreturn json + JSON_STR_END_OBJECT\n'
: '\njson += JSON_STR_END_OBJECT\n'
return code
}

Expand Down Expand Up @@ -594,6 +628,140 @@ function toJSON (variableName) {
`
}

const SINGLE_EXPRESSION_STATEMENT = /^json \+= (.*)$/
const JS_STRING_LITERAL = /^"(?:[^"\\]|\\.)*"$/

// Values of the JSON_STR_* constants defined in the generated code preamble,
// so single-expression statements using them can be folded into literals.
const JSON_STR_CONSTANTS = {
JSON_STR_NULL: 'null',
JSON_STR_EMPTY_STRING: '""',
JSON_STR_EMPTY_OBJECT: '{}',
JSON_STR_EMPTY_ARRAY: '[]',
JSON_STR_QUOTE: '"'
}

// If the generated code is a single `json += <expression>` statement,
// return the expression so it can be merged with a prefix into a single
// string concatenation. Otherwise return null.
function extractSingleExpression (code) {
const trimmed = code.trim()
if (trimmed.indexOf('\n') !== -1) return null
const match = trimmed.match(SINGLE_EXPRESSION_STATEMENT)
return match === null ? null : match[1]
}

// Folds two compile-time string constants into a single literal, or returns
// null if either side is not a constant. Runtime concatenation of a literal
// with a non-constant expression is intentionally never generated here: a
// separate `json += <literal>` statement takes V8's string-append fast path,
// while `json += lit + call()` measures ~20% slower in hot loops.
function foldStringConstants (prefixExpr, valueExpr) {
if (JSON_STR_CONSTANTS[valueExpr] !== undefined) {
valueExpr = JSON.stringify(JSON_STR_CONSTANTS[valueExpr])
}
if (JS_STRING_LITERAL.test(prefixExpr) && JS_STRING_LITERAL.test(valueExpr)) {
return JSON.stringify(JSON.parse(prefixExpr) + JSON.parse(valueExpr))
}
return null
}

function emitPrefixed (prefixExpr, code) {
const expr = extractSingleExpression(code)
if (expr !== null) {
const folded = foldStringConstants(prefixExpr, expr)
if (folded !== null) {
return `json += ${folded}`
}
}
return `json += ${prefixExpr}\n${code}`
}

function buildSingleTypeWithPrefix (context, location, input, prefixExpr) {
return emitPrefixed(prefixExpr, buildSingleTypeSerializer(context, location, input))
}

// A non-recursive plain object schema that is not shared through a $ref can
// be serialized inline into the parent's json accumulator, avoiding a
// function call and an intermediate string per object. Each schema is
// inlined at most once so shared schema objects cannot blow up code size.
function isInlinableObjectSchema (context, location) {
const schema = location.schema
return typeof schema === 'object' &&
schema !== null &&
schema.type === 'object' &&
schema.const === undefined &&
schema.allOf === undefined &&
schema.anyOf === undefined &&
schema.oneOf === undefined &&
!(schema.if && schema.then) &&
!context.recursivePaths.has(`${location.schemaId || ''}#${location.jsonPointer || ''}`) &&
!context.buildingSet.has(schema) &&
!context.functionsNamesBySchema.has(schema) &&
!context.inlinedSchemas.has(schema)
}

function buildInlineObjectValue (context, location, input) {
const schema = location.schema
const nullable = schema.nullable === true

context.inlinedSchemas.add(schema)
context.buildingSet.add(schema)
const objVar = `obj_${context.uid++}`
const code = `
const ${objVar} = ${toJSON(input)}
if (${objVar} === null) {
json += ${nullable ? 'JSON_STR_NULL' : 'JSON_STR_EMPTY_OBJECT'}
} else {
${buildInnerObject(context, location, objVar)}
}
`
context.buildingSet.delete(schema)
return code
}

// Like buildValue, but emits a constant prefix (object key, optionally with
// leading `{` or `,`) as a single pre-folded literal statement, folding it
// into the value too when the value is itself a compile-time constant.
// allowInline permits serializing a plain nested object schema inline; it
// must only be set when the schema was not reached through a $ref.
function buildValueWithPrefix (context, location, input, prefixExpr, allowInline) {
let schema = location.schema

if (typeof schema === 'object' && schema !== null) {
if (schema.$ref !== undefined) {
location = resolveRef(context, location)
schema = location.schema
allowInline = false
}

if (schema.type === undefined) {
const inferredType = inferTypeByKeyword(schema)
if (inferredType) {
schema.type = inferredType
}
}

if (
Array.isArray(schema.type) &&
schema.const === undefined &&
schema.nullable !== true &&
schema.allOf === undefined &&
schema.anyOf === undefined &&
schema.oneOf === undefined &&
!(schema.if && schema.then)
) {
return buildMultiTypeSerializer(context, location, input, prefixExpr)
}

if (allowInline && isInlinableObjectSchema(context, location)) {
return `json += ${prefixExpr}\n${buildInlineObjectValue(context, location, input)}`
}
}

return emitPrefixed(prefixExpr, buildValue(context, location, input))
}

function buildObject (context, location, input) {
const schema = location.schema

Expand Down Expand Up @@ -621,8 +789,7 @@ function buildObject (context, location, input) {
if (obj === null) return ${nullable ? 'JSON_STR_NULL' : 'JSON_STR_EMPTY_OBJECT'}
let json = ''

${buildInnerObject(context, location, 'obj')}
return json
${buildInnerObject(context, location, 'obj', true)}
}
`

Expand Down Expand Up @@ -650,7 +817,8 @@ function buildArray (context, location, input) {
let itemsLocation = location.getPropertyLocation('items')
itemsLocation.schema = itemsLocation.schema || {}

if (itemsLocation.schema.$ref) {
const itemsHadRef = itemsLocation.schema.$ref !== undefined
if (itemsHadRef) {
itemsLocation = resolveRef(context, itemsLocation)
}

Expand Down Expand Up @@ -738,7 +906,9 @@ function buildArray (context, location, input) {
}`
}
} else {
const code = buildValue(context, itemsLocation, 'value')
const code = (!itemsHadRef && isInlinableObjectSchema(context, itemsLocation))
? buildInlineObjectValue(context, itemsLocation, 'value')
: buildValue(context, itemsLocation, 'value')
functionCode += `
if (arrayLength > 0) {
const value = obj[0]
Expand Down Expand Up @@ -894,15 +1064,17 @@ function generateFuncName (context) {
return 'anonymous' + context.functionsCounter++
}

function buildMultiTypeSerializer (context, location, input) {
function buildMultiTypeSerializer (context, location, input, prefixExpr) {
const schema = location.schema
const types = schema.type.sort(t1 => t1 === 'null' ? -1 : 1)

let code = ''

types.forEach((type, index) => {
location.schema = { ...location.schema, type }
const nestedResult = buildSingleTypeSerializer(context, location, input)
const nestedResult = prefixExpr === undefined
? buildSingleTypeSerializer(context, location, input)
: buildSingleTypeWithPrefix(context, location, input, prefixExpr)

const statement = index === 0 ? 'if' : 'else if'
switch (type) {
Expand Down Expand Up @@ -1013,7 +1185,7 @@ function buildSingleTypeSerializer (context, location, input) {
case 'number':
return `json += asNumber(${input})`
case 'boolean':
return `json += asBoolean(${input})`
return `json += (${input} ? 'true' : 'false')`
case 'object': {
return buildObject(context, location, input)
}
Expand Down