# Builtin System Design

## Overview

Type-safe builtin system for exposing Go functions to TypeScript/JavaScript with automatic type conversion and defaults support.

## Core Design

### Single Argument Pattern

All builtins accept a **single struct argument** on the Go side, but are called as **multi-argument functions** on the JavaScript side.

```go
type FetchArgs struct {
    URL     string `json:"url"`
    Options *FetchOptions `json:"options"`
}

func Fetch(args FetchArgs) (*FetchResult, error) {
    // Implementation
}

// JavaScript calls:
// fetch("https://example.com")
// fetch("https://example.com", { method: "POST" })
```

**Mapping Rules:**
- JavaScript arguments map **positionally** to struct fields in order
- Field names in JavaScript come from `json:""` struct tag, or field name if tag is omitted
- Missing JavaScript arguments only allowed for **pointer fields** (non-pointer fields must always be provided)

### Default Values

Argument structs **must** implement `Defaults()` receiver method to provide default values:

```go
type FetchOptions struct {
    Method  string `json:"method"`
    Headers map[string]string `json:"headers"`
}

func (f *FetchOptions) Defaults() *FetchOptions {
    if f.Method == "" {
        f.Method = "GET"
    }
    return f
}

// If called with just URL, Options will be nil initially,
// then we create default: &FetchOptions{Method: "GET"}
```

**Defaults flow:**
1. Create zero-value struct
2. Fill in provided JavaScript arguments positionally
3. Call `Defaults()` on the struct to fill in remaining defaults

### Empty Arguments

Zero-argument builtins must still use a struct (can be empty):

```go
// Helper type for no-argument builtins
type EmptyArgs struct {}

func Ping(args EmptyArgs) bool {
    return true
}

// JavaScript: ping()
```

## Registration API

```go
package builtin

// RegisterBuiltin registers a Go function as a builtin
// The function must accept a single struct argument T and return (R, error) or R
func RegisterBuiltin[T any, R any](name string, fn func(T) (R, error))
```

**Usage:**
```go
func init() {
    builtin.RegisterBuiltin("fetch", Fetch)
    builtin.RegisterBuiltin("add", Add)
}
```

## Type Conversion

### JavaScript → Go

Automatic conversion from goja values to Go struct fields:

| JavaScript | Go Type |
|------------|---------|
| string | string |
| number | int, int8-64, uint, uint8-64, float32, float64 |
| boolean | bool |
| object/map | map[string]any |
| null/undefined | nil (for pointer fields only) |

**Struct Field Conversion:**
- Primitives: converted by type
- Maps: `map[string]any` → Go map with string keys
- Nested structs: Recursive conversion to nested struct

### Go → JavaScript (Return Types)

Automatic conversion from Go return values to JavaScript:

| Go Type | JavaScript |
|---------|------------|
| string, number, bool | primitive |
| map[string]any | object |
| map[string]string | object |
| []any | array |
| struct | object with fields from json tags |

**Struct Return Types:**
- Auto-converted to JavaScript objects
- Field names from `json:""` struct tags
- Nested structs become nested objects
- No custom converters needed

**Example:**
```go
type FetchResult struct {
    OK      bool   `json:"ok"`
    Status  int    `json:"status"`
    Body    string `json:"body"`
}

// Returns: { ok: true, status: 200, body: "..." }
```

## Error Handling

Go errors are automatically converted to JavaScript exceptions:

```go
func Fetch(args FetchArgs) (*FetchResult, error) {
    if someError {
        return nil, fmt.Errorf("fetch failed")
    }
    return result, nil
}

// JavaScript: fetch calls that error will throw
```

## TypeScript Declaration Generation

TypeScript definitions are auto-generated from Go function signatures:

```go
// Go:
func Fetch(args FetchArgs) (*FetchResult, error)

// Generated TypeScript:
declare function fetch(url: string, options?: FetchOptions): FetchResult | never;
```

**Type mapping:**
- `string` → `string`
- `int/int8-64/uint/uint8-64/float32/float64` → `number`
- `bool` → `boolean`
- `*T` → `T | null` (or `T` if optional)
- `map[string]any` → `Record<string, any>`
- `struct` → Interface with fields

**Optional parameters:**
- Pointer fields become optional parameters with `T | null`
- Non-pointer fields are required

## Implementation Notes

### Reflection-Based Wrapping

```go
func createWrapper[T any, R any](fn func(T) (R, error)) func(*goja.Runtime, goja.FunctionCall) goja.Value {
    return func(vm *goja.Runtime, call goja.FunctionCall) goja.Value {
        // 1. Create zero-value struct
        var args T
        argsValue := reflect.ValueOf(&args).Elem()

        // 2. Map JavaScript args to struct fields positionally
        for i := 0; i < min(len(call.Arguments), argsValue.NumField()); i++ {
            jsArg := call.Arguments[i]
            field := argsValue.Field(i)

            // Skip nil/undefined if field is pointer
            if goja.IsUndefined(jsArg) || goja.IsNull(jsArg) {
                if field.Kind() == reflect.Ptr {
                    continue
                }
            }

            // Convert and assign
            converted, err := convertJSValueToGo(vm, jsArg, field.Type())
            if err != nil {
                panic(err)
            }
            field.Set(reflect.ValueOf(converted))
        }

        // 3. Call Defaults() if defined
        if defaults, ok := args.(interface{ Defaults() T }); ok {
            args = defaults.Defaults()
        }

        // 4. Call the Go function
        result, err := fn(args)
        if err != nil {
            panic(err)
        }

        // 5. Convert return value to JavaScript
        return convertGoValueToJS(vm, reflect.ValueOf(result))
    }
}
```

### Field Name Extraction

```go
func getFieldName(field reflect.StructField) string {
    jsonTag := field.Tag.Get("json")
    if jsonTag != "" && jsonTag != "-" {
        name, _, _ := strings.Cut(jsonTag, ",")
        return name
    }
    return field.Name
}
```

## Migration Examples

### Old → New

**Old (current system):**
```go
func add(a, b int) int {
    return a + b
}

// Register: builtin.RegisterBuiltin("add", add)
// JS: add(5, 10)
```

**New system:**
```go
type AddArgs struct {
    A int `json:"a"`
    B int `json:"b"`
}

func add(args AddArgs) int {
    return args.A + args.B
}

// Register: builtin.RegisterBuiltin("add", add)
// JS: add(5, 10)          // positional
// JS: add(a=5, b=10)      // named (if supported)
```

**Old fetch:**
```go
func Fetch(url string, options map[string]any) (*FetchResult, error)

// Requires custom converter for result
```

**New fetch:**
```go
type FetchArgs struct {
    URL     string `json:"url"`
    Options *FetchOptions `json:"options"`
}

type FetchOptions struct {
    Method  string `json:"method"`
    Headers map[string]string `json:"headers"`
}

func (o *FetchOptions) Defaults() *FetchOptions {
    if o.Method == "" {
        o.Method = "GET"
    }
    return o
}

type FetchResult struct {
    OK      bool   `json:"ok"`
    Status  int    `json:"status"`
    Body    string `json:"body"`
}

func Fetch(args FetchArgs) (*FetchResult, error) {
    // Implementation - no custom converter needed
}

// JS: fetch("https://example.com")
// JS: fetch("https://example.com", { method: "POST" })
```

## Testing

All existing tests must pass after migration:

- `internal/runtime/runtime_test.go`
- `internal/runtime/standard/fetch_test.go`
- `test_data/*.ts` test files

Verify:
- TypeScript definitions are correct
- Positional argument mapping works
- Defaults are applied correctly
- Named JSON tags work
- Error handling propagates
- Return value conversion works