API Reference

TypeScript/JavaScript API documentation for the @toon-format/toon package. For format rules, see the Format Overview or the Specification. For other languages, see Implementations.

Installation

npm

npm install @toon-format/toon

pnpm

pnpm add @toon-format/toon

yarn

yarn add @toon-format/toon

Encoding Functions

encode(input, options?)

Converts any JSON-serializable value to TOON format.

import { encode } from '@toon-format/toon'

const toon = encode(data, {
  indent: 2,
  delimiter: ',',
  keyFolding: 'off',
  flattenDepth: Infinity
})

Parameters

Parameter	Type	Description
input	unknown	Any JSON-serializable value (object, array, primitive, or nested structure)
options	EncodeOptions?	Optional encoding options (see Configuration Reference)

Return Value

Returns a TOON-formatted string with no trailing newline or spaces.

Type Normalization

Non-JSON-serializable values are normalized before encoding:

Input	Output
Object with toJSON() method	Result of calling toJSON(), recursively normalized
Finite number	Canonical decimal (no exponent, no leading/trailing zeros: 1e6 → 1000000, -0 → 0)
NaN, Infinity, -Infinity	null
BigInt (within safe range)	Number
BigInt (out of range)	Quoted decimal string (e.g., "9007199254740993")
Date	ISO string in quotes (e.g., "2025-01-01T00:00:00.000Z")
undefined, function, symbol	null

Example

import { encode } from '@toon-format/toon'

const items = [
  { sku: 'A1', qty: 2, price: 9.99 },
  { sku: 'B2', qty: 1, price: 14.5 }
]

console.log(encode({ items }))

Output:

items[2]{sku,qty,price}:
  A1,2,9.99
  B2,1,14.5

encodeLines(input, options?)

Preferred method for streaming TOON output. Converts any JSON-serializable value to TOON format as a sequence of lines, without building the full string in memory. Suitable for streaming large outputs to files, HTTP responses, or process stdout.

import { encodeLines } from '@toon-format/toon'

// Stream to stdout (Node.js)
for (const line of encodeLines(data)) {
  process.stdout.write(`${line}\n`)
}

// Write to file line-by-line
const lines = encodeLines(data, { indent: 2, delimiter: '\t' })
for (const line of lines) {
  await writeToStream(`${line}\n`)
}

// Collect to array
const lineArray = Array.from(encodeLines(data))

Parameters

Parameter	Type	Description
input	unknown	Any JSON-serializable value (object, array, primitive, or nested structure)
options	EncodeOptions?	Optional encoding options (see Configuration Reference)

Return Value

Returns an Iterable<string> that yields TOON lines one at a time. Each yielded string is a single line without a trailing newline character — you must add \n when writing to streams or stdout.

Relationship to encode()

encode(value, options) is equivalent to:

Array.from(encodeLines(value, options)).join('\n')

Example

import { createWriteStream } from 'node:fs'
import { encodeLines } from '@toon-format/toon'

const data = {
  items: Array.from({ length: 100000 }, (_, i) => ({
    id: i,
    name: `Item ${i}`,
    value: Math.random()
  }))
}

// Stream large dataset to file
const stream = createWriteStream('output.toon')
for (const line of encodeLines(data, { delimiter: '\t' })) {
  stream.write(`${line}\n`)
}
stream.end()

Replacer Function

The replacer option allows you to transform or filter values during encoding. It works similarly to JSON.stringify's replacer parameter, but with path tracking for more precise control.

Type Signature

type EncodeReplacer = (
  key: string,
  value: JsonValue,
  path: readonly (string | number)[]
) => unknown

Parameters

Parameter	Type	Description
key	string	Property name, array index (as string), or empty string for root
value	JsonValue	The normalized value at this location
path	readonly (string | number)[]	Path from root to current value

Return Value

• Return the value unchanged to keep it
• Return a different value to replace it (will be normalized)
• Return undefined to omit properties/array elements
• For root value, undefined means "no change" (root cannot be omitted)

Examples

Filtering sensitive data:

import { encode } from '@toon-format/toon'

const data = {
  user: { name: 'Alice', password: 'secret123', email: 'alice@example.com' }
}

function replacer(key, value) {
  if (key === 'password')
    return undefined
  return value
}

console.log(encode(data, { replacer }))

Output:

user:
  name: Alice
  email: alice@example.com

Transforming values:

const data = { user: 'alice', role: 'admin' }

function replacer(key, value) {
  if (typeof value === 'string')
    return value.toUpperCase()
  return value
}

console.log(encode(data, { replacer }))

Output:

user: ALICE
role: ADMIN

Path-based transformations:

const data = {
  metadata: { created: '2025-01-01' },
  user: { created: '2025-01-02' }
}

function replacer(key, value, path) {
  // Add timezone info only to top-level metadata
  if (path.length === 1 && path[0] === 'metadata' && key === 'created') {
    return `${value}T00:00:00Z`
  }
  return value
}

console.log(encode(data, { replacer }))

Output:

metadata:
  created: 2025-01-01T00:00:00Z
user:
  created: 2025-01-02

Replacer Execution Order

The replacer is called in a depth-first manner:

1. Root value first (key = '', path = [])
2. Then each property/element (with proper key and path)
3. Values are re-normalized after replacement
4. Children are processed after parent transformation

Array Indices as Strings

Following JSON.stringify behavior, array indices are passed as strings ('0', '1', '2', etc.) to the replacer, not as numbers.

Decoding Functions

decode(input, options?)

Converts a TOON-formatted string back to JavaScript values.

import { decode } from '@toon-format/toon'

const data = decode(toon, {
  indent: 2,
  strict: true,
  expandPaths: 'off'
})

Parameters

Parameter	Type	Description
input	string	A TOON-formatted string to parse
options	DecodeOptions?	Optional decoding options (see Configuration Reference)

Return Value

Returns a JavaScript value (object, array, or primitive) representing the parsed TOON data.

Example

import { decode } from '@toon-format/toon'

const toon = `
items[2]{sku,qty,price}:
  A1,2,9.99
  B2,1,14.5
`

const data = decode(toon)
console.log(data)

Output:

{
  "items": [
    { "sku": "A1", "qty": 2, "price": 9.99 },
    { "sku": "B2", "qty": 1, "price": 14.5 }
  ]
}

decodeFromLines(lines, options?)

Decodes TOON format from pre-split lines into a JavaScript value. This is a streaming-friendly wrapper around the event-based decoder that builds the full value in memory.

Useful when you already have lines as an array or iterable (e.g., from file streams, readline interfaces, or network responses) and want the standard decode behavior with path expansion support.

Parameters

Parameter	Type	Description
lines	Iterable<string>	Iterable of TOON lines (without trailing newlines)
options	DecodeOptions?	Optional decoding configuration (see Configuration Reference)

Return Value

Returns a JsonValue (the parsed JavaScript value: object, array, or primitive).

Example

Basic usage with arrays:

import { decodeFromLines } from '@toon-format/toon'

const lines = ['name: Alice', 'age: 30']
const value = decodeFromLines(lines)
// { name: 'Alice', age: 30 }

Streaming from Node.js readline:

import { createReadStream } from 'node:fs'
import { createInterface } from 'node:readline'
import { decodeFromLines } from '@toon-format/toon'

const rl = createInterface({
  input: createReadStream('data.toon'),
  crlfDelay: Infinity,
})

const value = decodeFromLines(rl)
console.log(value)

With path expansion:

const lines = ['user.name: Alice', 'user.age: 30']
const value = decodeFromLines(lines, { expandPaths: 'safe' })
// { user: { name: 'Alice', age: 30 } }

Choosing the Right Decoder

Function	Input	Output	Async	Path Expansion	Use When
decode()	String	Value	No	Yes	You have a complete TOON string
decodeFromLines()	Lines	Value	No	Yes	You have lines and want the full value
decodeStreamSync()	Lines	Events	No	No	You need event-by-event processing (sync)
decodeStream()	Lines	Events	Yes	No	You need event-by-event processing (async)

Key Differences

• Value vs. Events: Functions ending in Stream yield events without building the full value in memory.
• Path expansion: Only decode() and decodeFromLines() support expandPaths: 'safe'.
• Async support: Only decodeStream() accepts async iterables (useful for file/network streams).

Streaming Decoders

decodeStreamSync(lines, options?)

Synchronously decodes TOON lines into a stream of JSON events. This function yields structured events that represent the JSON data model without building the full value tree.

Useful for streaming processing, custom transformations, or memory-efficient parsing of large datasets where you don't need the full value in memory.

Event Streaming

This is a low-level API that returns individual parse events. For most use cases, decodeFromLines() or decode() are more convenient.

Path expansion (expandPaths: 'safe') is not supported in streaming mode since it requires the full value tree.

Parameters

Parameter	Type	Description
lines	Iterable<string>	Iterable of TOON lines (without trailing newlines)
options	DecodeStreamOptions?	Optional streaming decoding configuration (see Configuration Reference)

Return Value

Returns an Iterable<JsonStreamEvent> that yields structured events (see TypeScript Types for event structure).

Example

Basic event streaming:

import { decodeStreamSync } from '@toon-format/toon'

const lines = ['name: Alice', 'age: 30']

for (const event of decodeStreamSync(lines)) {
  console.log(event)
}

// Output:
// { type: 'startObject' }
// { type: 'key', key: 'name' }
// { type: 'primitive', value: 'Alice' }
// { type: 'key', key: 'age' }
// { type: 'primitive', value: 30 }
// { type: 'endObject' }

Custom processing:

import { decodeStreamSync } from '@toon-format/toon'

const lines = ['users[2]{id,name}:', '  1,Alice', '  2,Bob']
let userCount = 0

for (const event of decodeStreamSync(lines)) {
  if (event.type === 'endObject' && userCount < 2) {
    userCount++
    console.log(`Processed user ${userCount}`)
  }
}

decodeStream(source, options?)

Asynchronously decodes TOON lines into a stream of JSON events. This is the async version of decodeStreamSync(), supporting both synchronous and asynchronous iterables.

Useful for processing file streams, network responses, or other async sources where you want to handle data incrementally as it arrives.

Parameters

Parameter	Type	Description
source	AsyncIterable<string> | Iterable<string>	Async or sync iterable of TOON lines (without trailing newlines)
options	DecodeStreamOptions?	Optional streaming decoding configuration (see Configuration Reference)

Return Value

Returns an AsyncIterable<JsonStreamEvent> that yields structured events asynchronously (see TypeScript Types for event structure).

Example

Streaming from file:

import { createReadStream } from 'node:fs'
import { createInterface } from 'node:readline'
import { decodeStream } from '@toon-format/toon'

const fileStream = createReadStream('data.toon', 'utf-8')
const rl = createInterface({ input: fileStream, crlfDelay: Infinity })

for await (const event of decodeStream(rl)) {
  console.log(event)
  // Process events as they arrive
}

Configuration Reference

EncodeOptions

Configuration for encode() and encodeLines():

Option	Type	Default	Description
indent	number	2	Number of spaces per indentation level
delimiter	',' | '\t' | '|'	','	Delimiter for array values and tabular rows
keyFolding	'off' | 'safe'	'off'	Enable key folding to collapse single-key wrapper chains into dotted paths
flattenDepth	number	Infinity	Maximum number of segments to fold when keyFolding is enabled (values 0-1 have no practical effect)
replacer	EncodeReplacer	undefined	Optional hook to transform or omit values before encoding (see Replacer Function)

Delimiter options:

Comma (default)

encode(data, { delimiter: ',' })

Tab

encode(data, { delimiter: '\t' })

Pipe

encode(data, { delimiter: '|' })

See Delimiter Strategies for guidance on choosing delimiters.

DecodeOptions

Configuration for decode() and decodeFromLines():

Option	Type	Default	Description
indent	number	2	Expected number of spaces per indentation level
strict	boolean	true	Enable strict validation (array counts, indentation, delimiter consistency)
expandPaths	'off' | 'safe'	'off'	Enable path expansion to reconstruct dotted keys into nested objects (pairs with keyFolding: 'safe')

By default (strict: true), the decoder validates input strictly:

• Invalid escape sequences: Throws on \x, unterminated strings
• Syntax errors: Throws on missing colons, malformed headers
• Array length mismatches: Throws when declared length doesn't match actual count
• Delimiter mismatches: Throws when row delimiters don't match header
• Indentation errors: Throws when leading spaces aren't exact multiples of indent

Set strict: false to skip validation for lenient parsing.

See Key Folding & Path Expansion for more details on path expansion behavior and conflict resolution.

DecodeStreamOptions

Configuration for decodeStreamSync() and decodeStream():

Option	Type	Default	Description
indent	number	2	Expected number of spaces per indentation level
strict	boolean	true	Enable strict validation (array counts, indentation, delimiter consistency)

Path Expansion Not Supported

Path expansion requires building the full value tree, which is incompatible with event streaming. Use decodeFromLines() if you need path expansion.

TypeScript Types

JsonStreamEvent

Events emitted by decodeStreamSync() and decodeStream():

type JsonStreamEvent
  = | { type: 'startObject' }
    | { type: 'endObject' }
    | { type: 'startArray', length: number }
    | { type: 'endArray' }
    | { type: 'key', key: string, wasQuoted?: boolean }
    | { type: 'primitive', value: JsonPrimitive }

type JsonPrimitive = string | number | boolean | null

Guides & Examples

Round-Trip Compatibility

TOON provides lossless round-trips after normalization:

import { decode, encode } from '@toon-format/toon'

const original = {
  users: [
    { id: 1, name: 'Alice', role: 'admin' },
    { id: 2, name: 'Bob', role: 'user' }
  ]
}

const toon = encode(original)
const restored = decode(toon)

console.log(JSON.stringify(original) === JSON.stringify(restored))
// true

With Key Folding:

import { decode, encode } from '@toon-format/toon'

const original = { data: { metadata: { items: ['a', 'b'] } } }

// Encode with folding
const toon = encode(original, { keyFolding: 'safe' })
// → "data.metadata.items[2]: a,b"

// Decode with expansion
const restored = decode(toon, { expandPaths: 'safe' })
// → { data: { metadata: { items: ['a', 'b'] } } }

console.log(JSON.stringify(original) === JSON.stringify(restored))
// true

Key Folding & Path Expansion

Key Folding (keyFolding: 'safe') collapses single-key wrapper chains during encoding:

import { encode } from '@toon-format/toon'

const data = { data: { metadata: { items: ['a', 'b'] } } }

// Without folding
encode(data)
// data:
//   metadata:
//     items[2]: a,b

// With folding
encode(data, { keyFolding: 'safe' })
// data.metadata.items[2]: a,b

Path Expansion (expandPaths: 'safe') reverses this during decoding:

import { decode } from '@toon-format/toon'

const toon = 'data.metadata.items[2]: a,b'

const data = decode(toon, { expandPaths: 'safe' })
console.log(data)
// { data: { metadata: { items: ['a', 'b'] } } }

Expansion Conflict Resolution:

When multiple expanded keys construct overlapping paths, the decoder merges them recursively:

• Object + Object: Deep merge recursively
• Object + Non-object (array or primitive): Conflict
  • With strict: true (default): Error
  • With strict: false: Last-write-wins (LWW)

Delimiter Strategies

Tab delimiters (\t) often tokenize more efficiently than commas, as Tabs are single characters that rarely appear in natural text. This reduces the need for quote-escaping, leading to smaller token counts in large datasets.

Example:

items[2	]{sku	name	qty	price}:
  A1	Widget	2	9.99
  B2	Gadget	1	14.5

For maximum token savings on large tabular data, combine tab delimiters with key folding:

encode(data, { delimiter: '\t', keyFolding: 'safe' })

Choosing a Delimiter:

• Comma (,): Default, widely understood, good for simple tabular data.
• Tab (\t): Best for LLM token efficiency, excellent for large datasets.
• Pipe (|): Alternative when commas appear frequently in data.