toon-format--toon/docs/reference/api.md

---
description: TypeScript and JavaScript encode and decode functions, options, error types, and streaming decoders for @toon-format/toon.
---

# API Reference

TypeScript/JavaScript API documentation for the `@toon-format/toon` package. For format rules, see the [Format Overview](/guide/format-overview) or the [Specification](/reference/spec). For other languages, see [Implementations](/ecosystem/implementations).

## Installation

::: code-group

```bash [npm]
npm install @toon-format/toon
```

```bash [pnpm]
pnpm add @toon-format/toon
```

```bash [yarn]
yarn add @toon-format/toon
```

:::

## Encoding Functions

### `encode(input, options?)`

Converts any JSON-serializable value to TOON format.

```ts
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](#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 in `[1e-6, 1e21)` (or zero) | Canonical decimal (e.g., `1e6` → `1000000`, `-0` → `0`) |
| Finite number outside that range | Exponent form permitted (e.g., `1e-7`, `1e+21`) |
| `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"`) |
| `Set` | Array of normalized values |
| `Map` | Object with `String(key)` keys |
| `undefined`, `function`, `symbol` | `null` |

::: info
TOON itself doesn't specify how `Date` should be encoded – the spec leaves this to implementations. This library emits an ISO 8601 string in quotes; other implementations may choose differently.
:::

#### Example

```ts
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:**

```yaml
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.

```ts
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](#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.

::: info Relationship to `encode()`
`encode(value, options)` is equivalent to:
```ts
Array.from(encodeLines(value, options)).join('\n')
```
:::

#### Example

```ts
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

```typescript
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:**

```typescript
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:**

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

**Transforming values:**

```typescript
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:**

```yaml
user: ALICE
role: ADMIN
```

**Path-based transformations:**

```typescript
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:**

```yaml
metadata:
  created: "2025-01-01T00:00:00Z"
user:
  created: 2025-01-02
```

::: info 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
:::

::: warning 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.

```ts
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](#configuration-reference)) |

#### Return Value

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

#### Example

```ts
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:**

```json
{
  "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](#configuration-reference)) |

#### Return Value

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

#### Example

**Basic usage with arrays:**

```ts
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:**

```ts
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:**

```ts
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) |

::: info 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.

::: tip Event Streaming
This is a low-level API that returns individual parse events. For most use cases, [`decodeFromLines()`](#decodefromlines-lines-options) or [`decode()`](#decode-input-options) 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](#configuration-reference)) |

#### Return Value

Returns an `Iterable<JsonStreamEvent>` that yields structured events (see [TypeScript Types](#typescript-types) for event structure).

#### Example

**Basic event streaming:**

```ts
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:**

```ts
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()`](#decodestreamsync-lines-options), 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](#configuration-reference)) |

#### Return Value

Returns an `AsyncIterable<JsonStreamEvent>` that yields structured events asynchronously (see [TypeScript Types](#typescript-types) for event structure).

#### Example

**Streaming from file:**

```ts
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
}
```

## Error Handling

Decoding throws a `ToonDecodeError` when input cannot be parsed. The class extends `SyntaxError`, so existing `error instanceof SyntaxError` checks keep working without code changes.

### `ToonDecodeError`

```ts
import { ToonDecodeError } from '@toon-format/toon'
```

#### Fields

| Field | Type | Description |
|-------|------|-------------|
| `name` | `'ToonDecodeError'` | Discriminator – `error.name === 'ToonDecodeError'` |
| `message` | `string` | Human-readable message; prefixed with `Line N: ` when a line is known |
| `line` | `number?` | 1-based line number where the error was detected |
| `source` | `string?` | Raw source line (including its leading whitespace) |
| `cause` | `unknown?` | The original error when the decoder enriched a lower-level parser failure |

The `line` and `source` fields are populated for every error that has line context – essentially every parse error during normal decoding. The `cause` chain points back to the underlying `SyntaxError` or `TypeError` thrown by the token-level parser, so debuggers and verbose loggers can show the original frame.

#### Example

```ts
import { decode, ToonDecodeError } from '@toon-format/toon'

try {
  decode('a:\n\tb: 1')
}
catch (error) {
  if (error instanceof ToonDecodeError) {
    console.error(`Line ${error.line}:`, error.source)
    console.error(error.message)
    // Line 2: 	b: 1
    // Line 2: Tabs are not allowed in indentation in strict mode
  }
  else {
    throw error
  }
}
```

::: info Backwards Compatibility
`ToonDecodeError` extends `SyntaxError`. Code written against earlier versions that catches `SyntaxError` continues to match these errors. The class adds structured fields without removing anything.
:::

## Configuration Reference

### `EncodeOptions`

Configuration for [`encode()`](#encode-input-options) and [`encodeLines()`](#encodelines-input-options):

| 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](#replacer-function)) |

**Delimiter options:**

::: code-group

```ts [Comma (default)]
encode(data, { delimiter: ',' })
```

```ts [Tab]
encode(data, { delimiter: '\t' })
```

```ts [Pipe]
encode(data, { delimiter: '|' })
```

:::

See [Delimiter Strategies](#delimiter-strategies) for guidance on choosing delimiters.

### `DecodeOptions`

Configuration for [`decode()`](#decode-input-options) and [`decodeFromLines()`](#decodefromlines-lines-options):

| 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, lone-surrogate `\uXXXX`
- **Syntax errors**: Throws on missing colons, malformed headers
- **Array length mismatches**: Throws when declared length doesn't match actual count
- **Header delimiter mismatch**: Throws when the bracket-declared delimiter differs from the field-list delimiter (§14.2)
- **Indentation errors**: Throws when leading spaces aren't exact multiples of `indent`
- **Header structure**: Throws on leading-zero or non-integer array lengths and on intervening content between bracket/fields/colon
- **Duplicate sibling keys**: Throws when an object has two children with the same key (§14.4)
- **Path-expansion conflicts**: When `expandPaths: 'safe'` is set, throws on overlapping dotted paths that would collide

All decode errors are thrown as [`ToonDecodeError`](#error-handling) instances with structured `line` and `source` fields.

Set `strict: false` to skip these checks. Duplicate sibling keys and path-expansion conflicts then resolve with last-write-wins in document order.

See [Key Folding & Path Expansion](#key-folding-path-expansion) for more details on path expansion behavior and conflict resolution.

### `DecodeStreamOptions`

Configuration for [`decodeStreamSync()`](#decodestreamsync-lines-options) and [`decodeStream()`](#decodestream-source-options):

| 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) |

::: warning Path Expansion Not Supported
Path expansion requires building the full value tree, which is incompatible with event streaming. Use [`decodeFromLines()`](#decodefromlines-lines-options) if you need path expansion.
:::

## TypeScript Types

### `JsonStreamEvent`

Events emitted by [`decodeStreamSync()`](#decodestreamsync-lines-options) and [`decodeStream()`](#decodestream-source-options):

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

### Delimiters

```ts
import { DEFAULT_DELIMITER, DELIMITERS } from '@toon-format/toon'

DEFAULT_DELIMITER // ','
DELIMITERS // { comma: ',', tab: '\t', pipe: '|' }
```

| Export | Description |
|--------|-------------|
| `DEFAULT_DELIMITER` | The default delimiter character (`,`) used when none is specified |
| `DELIMITERS` | Frozen record mapping delimiter names to their characters |
| `Delimiter` | Type union of valid delimiter characters: `',' \| '\t' \| '\|'` |
| `DelimiterKey` | Type union of delimiter names: `'comma' \| 'tab' \| 'pipe'` |

### Option Types

| Export | Description |
|--------|-------------|
| `EncodeOptions` | Options accepted by [`encode()`](#encode-input-options) and [`encodeLines()`](#encodelines-input-options) |
| `DecodeOptions` | Options accepted by [`decode()`](#decode-input-options) and [`decodeFromLines()`](#decodefromlines-lines-options) |
| `DecodeStreamOptions` | Options accepted by [`decodeStreamSync()`](#decodestreamsync-lines-options) and [`decodeStream()`](#decodestream-source-options) |
| `EncodeReplacer` | Signature of the [replacer function](#replacer-function) |
| `ResolvedEncodeOptions` | `EncodeOptions` after defaults are applied (advanced) |
| `ResolvedDecodeOptions` | `DecodeOptions` after defaults are applied (advanced) |

## Guides & Examples

### Round-Trip Compatibility

TOON provides lossless round-trips after normalization:

```ts
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:**

```ts
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:

```ts
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:

```ts
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)

Duplicate sibling keys (independent of `expandPaths`) follow the same policy: strict mode throws, lenient mode keeps the last value seen.

### Delimiter Strategies

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

Example:

```yaml
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:

```ts
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.