# JavaScript Interop

This document covers Baba Yaga's JavaScript interoperability features, which allow safe and controlled access to JavaScript functionality while maintaining Baba Yaga's functional programming guarantees.

## Table of Contents

1. [Overview](#overview)
2. [Core Functions](#core-functions)
3. [Type Conversion](#type-conversion)
4. [Security Model](#security-model)
5. [Common Patterns](#common-patterns)
6. [Error Handling](#error-handling)
7. [Configuration](#configuration)
8. [Best Practices](#best-practices)
9. [Examples](#examples)

## Overview

Baba Yaga's JavaScript interop system provides a safe bridge between Baba Yaga's functional, immutable world and JavaScript's imperative, mutable one. All JavaScript operations return `Result` types to maintain explicit error handling.

### Key Principles

- **Safety First**: All JS operations are sandboxed and return `Result` types
- **Explicit Boundaries**: Clear separation between Baba Yaga and JavaScript
- **Type Safety**: Automatic conversion between type systems
- **Error Isolation**: JavaScript errors become Baba Yaga `Err` values

## Core Functions

All JavaScript interop functions are available in the `io.*` namespace.

### Function Calls

#### `io.callJS`
Call a JavaScript function synchronously.

```baba
io.callJS : (functionName: String, args: [Any]) -> Result

// Examples
absResult : io.callJS "Math.abs" [-42];
// Returns: Ok (JSValue 42)

parseResult : io.callJS "JSON.parse" ["{\"x\": 10}"];
// Returns: Ok (JSValue {x: 10})

// Note: io.callJS returns a Result whose Ok value is a JSValue wrapper
// around the raw JavaScript value. You can pass this JSValue directly to
// io.getProperty, io.setProperty, io.hasProperty, io.jsArrayToList,
// io.objectToTable, etc. without manual unwrapping.
```

#### `io.callJSAsync`
Call a JavaScript function asynchronously (if async operations are enabled).

```baba
io.callJSAsync : (functionName: String, args: [Any]) -> Result

// Example (requires enableAsyncOps: true)
fetchResult : io.callJSAsync "fetch" ["https://api.example.com/data"];
```

### Property Access

#### `io.getProperty`
Get a property from a JavaScript object.

```baba
io.getProperty : (obj: Any, propName: String) -> Result

// Example
obj : io.callJS "JSON.parse" ["{\"name\": \"Alice\"}"];
nameResult : when obj is
  Ok parsed then io.getProperty parsed "name"
  Err msg then Err msg;
// Returns: Ok "Alice" (direct Baba Yaga string)
```

#### `io.setProperty`
Set a property on a JavaScript object (mutates the object).

```baba
io.setProperty : (obj: Any, propName: String, value: Any) -> Result

// Example
obj : io.callJS "JSON.parse" ["{}"];
result : when obj is
  Ok parsed then io.setProperty parsed "newProp" 42
  Err msg then Err msg;
```

#### `io.hasProperty`
Check if a property exists on a JavaScript object.

```baba
io.hasProperty : (obj: Any, propName: String) -> Bool

// Example
obj : io.callJS "JSON.parse" ["{\"x\": 10}"];
hasX : when obj is
  Ok parsed then io.hasProperty parsed "x"
  Err _ then false;
// Returns: true
```

### Type Conversion

#### `io.jsArrayToList`
Convert a JavaScript array to a Baba Yaga list.

```baba
io.jsArrayToList : (jsArray: Any) -> Result

// Example
jsArray : io.callJS "JSON.parse" ["[1, 2, 3]"];
listResult : when jsArray is
  Ok arr then io.jsArrayToList arr
  Err msg then Err msg;
// Returns: Ok [1, 2, 3] (direct Baba Yaga list)
```

#### `io.listToJSArray`
Convert a Baba Yaga list to a JavaScript array.

```baba
io.listToJSArray : (list: [Any]) -> Any

// Example
babaList : [1, 2, 3, 4, 5];
jsArray : io.listToJSArray babaList;
jsonResult : io.callJS "JSON.stringify" [jsArray];
// Returns: Ok (JSValue "[1,2,3,4,5]")
```

#### `io.objectToTable`
Convert a JavaScript object to a Baba Yaga table.

```baba
io.objectToTable : (obj: Any) -> Result

// Example
jsObj : io.callJS "JSON.parse" ["{\"name\": \"Bob\", \"age\": 25}"];
tableResult : when jsObj is
  Ok obj then io.objectToTable obj
  Err msg then Err msg;
// Returns: Ok {name: "Bob", age: 25} (direct Baba Yaga table)
```

#### `io.tableToObject`
Convert a Baba Yaga table to a JavaScript object.

```baba
io.tableToObject : (table: Table) -> Any

// Example
babaTable : {x: 100, y: 200};
jsObj : io.tableToObject babaTable;
jsonResult : io.callJS "JSON.stringify" [jsObj];
// Returns: Ok (JSValue "{\"x\":100,\"y\":200}")
```

### Error Management

#### `io.getLastJSError`
Get the last JavaScript error that occurred.

Note: depending on language syntax rules for zero-argument functions, direct invocation may not be available in all contexts. Prefer handling errors from `io.callJS` directly via the returned `Result`.

#### `io.clearJSError`
Clear the last JavaScript error.

Note: same invocation caveat as above applies.

## Type Conversion

### Automatic Conversions

The JavaScript bridge automatically converts between Baba Yaga and JavaScript types:

| Baba Yaga Type | JavaScript Type | Notes                               |
|----------------|-----------------|-------------------------------------|
| `Number`       | `number`        | Preserves integer/float distinction |
| `String`       | `string`        | Direct conversion                   |
| `Bool`         | `boolean`       | Direct conversion                   |
| `List`         | `Array`         | Recursive conversion of elements    |
| `Table`        | `Object`        | Converts Map to plain object        |
| `Result`       |  N/A            | Handled at boundary                 |


### Manual Conversions

For more control, use explicit conversion functions:

```baba
// Safe JSON parsing with error handling
parseJSON : jsonString ->
  when (validate.type "String" jsonString) is
    false then Err "Input must be a string"
    true then when (io.callJS "JSON.parse" [jsonString]) is
      Ok parsed then Ok (io.objectToTable parsed)
      Err msg then Err ("JSON parse error: " .. msg);

// Usage
result : parseJSON "{\"user\": \"Alice\", \"score\": 95}";
```

## Security Model

The JavaScript interop system uses a configurable security model:

### Sandboxed Execution

All JavaScript code runs in a controlled sandbox with:

- **Limited Global Access**: Only allowed globals are available
- **Function Whitelist**: Only explicitly allowed functions can be called
- **Timeout Protection**: Operations have configurable time limits
- **Memory Limits**: Configurable memory usage constraints

### Default Allowed Functions

By default, these JavaScript functions are available:

```javascript
// JSON operations
'JSON.parse', 'JSON.stringify',

// Math operations
'Math.abs', 'Math.floor', 'Math.ceil', 'Math.round',
'Math.min', 'Math.max', 'Math.random',

// Console operations
'console.log', 'console.warn', 'console.error',

// Time operations
'Date.now', 'performance.now'
```

### Configuration

Configure the JavaScript bridge through the host configuration:

```javascript
const host = {
  jsBridgeConfig: {
    allowedFunctions: new Set(['Math.abs', 'JSON.parse']),
    maxExecutionTime: 5000,  // 5 seconds
    enableAsyncOps: false,   // Disable async operations
    enableFileSystem: false, // Disable file system access
    enableNetwork: false     // Disable network access
  }
};
```

## Common Patterns

### Safe JSON Operations

```baba
// Safe JSON parsing
safeParseJSON : jsonStr ->
  when (io.callJS "JSON.parse" [jsonStr]) is
    Ok obj then when (io.objectToTable obj) is
      Ok table then Ok table
      Err msg then Err ("Conversion error: " .. msg)
    Err msg then Err ("Parse error: " .. msg);

// Safe JSON stringification
safeStringifyJSON : table ->
  jsObj : io.tableToObject table;
  io.callJS "JSON.stringify" [jsObj];
```

### Mathematical Operations

```baba
// Safe mathematical operations with validation
safeMath : operation args ->
  when (validate.notEmpty args) is
    false then Err "No arguments provided"
    true then when operation is
      "abs" then io.callJS "Math.abs" [head args]
      "min" then io.callJS "Math.min" args
      "max" then io.callJS "Math.max" args
      "round" then io.callJS "Math.round" [head args]
      _ then Err ("Unknown operation: " .. operation);

// Usage
result : safeMath "abs" [-42];  // Ok 42
minResult : safeMath "min" [10, 5, 8];  // Ok 5
```

### Working with JavaScript APIs

```baba
// Date operations
getCurrentTimestamp : () ->
  io.callJS "Date.now" [];

formatDate : timestamp ->
  when (io.callJS "Date" [timestamp]) is
    Ok dateObj then io.callJS "Date.prototype.toISOString" [dateObj]
    Err msg then Err msg;

// Performance monitoring
measurePerformance : operation ->
  startTime : io.callJS "performance.now" [];
  result : operation;
  endTime : io.callJS "performance.now" [];
  
  duration : when (startTime, endTime) is
    (Ok start, Ok end) then Ok (end - start)
    _ then Err "Could not measure performance";
  
  {result: result, duration: duration};
```

## Error Handling

### JavaScript Error Types

JavaScript errors are automatically converted to Baba Yaga `Err` values:

```baba
// This will return an Err
result : io.callJS "JSON.parse" ["invalid json"];
// Returns: Err "Unexpected token i in JSON at position 0"

// Handle different error types
handleJSError : result ->
  when result is
    Ok value then processValue value
    Err msg then when (text.contains msg "JSON") is
      true then handleJSONError msg
      false then handleGenericError msg;
```

### Error Recovery Patterns

```baba
// Retry pattern
retryOperation : operation maxAttempts ->
  attempt : 1;
  
  tryOperation : currentAttempt ->
    when (currentAttempt > maxAttempts) is
      true then Err "Max attempts exceeded"
      false then when (operation) is
        Ok result then Ok result
        Err _ then tryOperation (currentAttempt + 1);
  
  tryOperation attempt;

// Fallback pattern
withFallback : primaryOp fallbackOp ->
  when primaryOp is
    Ok result then Ok result
    Err _ then fallbackOp;
```

## Best Practices

### 1. Always Use Result Types

Never assume JavaScript operations will succeed:

```baba
// Good
result : when (io.callJS "Math.abs" [value]) is
  Ok abs then processValue abs
  Err msg then handleError msg;

// Bad - assumes success
abs : io.callJS "Math.abs" [value];  // This returns Result, not number
```

### 2. Validate Inputs

Always validate data before sending to JavaScript:

```baba
// Good
safeCall : value ->
  when (validate.type "Number" value) is
    false then Err "Value must be a number"
    true then io.callJS "Math.abs" [value];

// Bad - no validation
unsafeCall : value ->
  io.callJS "Math.abs" [value];
```

### 3. Handle Type Conversions Explicitly

Be explicit about type conversions:

```baba
// Good
processJSData : jsData ->
  when (io.objectToTable jsData) is
    Ok table then processTable table
    Err msg then Err ("Conversion failed: " .. msg);

// Bad - assumes conversion works
processJSData : jsData ->
  table : io.objectToTable jsData;
  processTable table;
```

### 4. Use Composition for Complex Operations

Break complex JavaScript interactions into smaller, composable functions:

```baba
// Composed operations
parseAndValidate : jsonStr schema ->
  parsed : safeParseJSON jsonStr;
  when parsed is
    Ok data then validateAgainstSchema data schema
    Err msg then Err msg;

transformAndStringify : data transformer ->
  transformed : transformer data;
  safeStringifyJSON transformed;
```

## Examples

### Complete JSON Processing Pipeline

```baba
// Complete JSON processing with error handling
processJSONData : jsonString ->
  // Parse JSON
  parseResult : io.callJS "JSON.parse" [jsonString];
  
  when parseResult is
    Err msg then Err ("Parse failed: " .. msg)
    Ok jsObj then 
      // Convert to Baba Yaga table
      when (io.objectToTable jsObj) is
        Err msg then Err ("Conversion failed: " .. msg)
        Ok table then
          // Process the data
          processedTable : processData table;
          
          // Convert back to JS object
          jsResult : io.tableToObject processedTable;
          
          // Stringify result
          io.callJS "JSON.stringify" [jsResult];

// Helper function
processData : table ->
  // Add timestamp
  withTimestamp : table .. {timestamp: getCurrentTimestamp};
  
  // Validate required fields
  when (hasRequiredFields withTimestamp) is
    false then table .. {error: "Missing required fields"}
    true then withTimestamp;

// Usage
input : "{\"name\": \"Alice\", \"score\": 95}";
result : processJSONData input;
// Returns: Ok (JSValue "{\"name\":\"Alice\",\"score\":95,\"timestamp\":1640995200000}")
```

### Working with JavaScript Arrays

```baba
// Process JavaScript arrays
processJSArray : jsArrayString ->
  // Parse array
  arrayResult : io.callJS "JSON.parse" [jsArrayString];
  
  when arrayResult is
    Err msg then Err msg
    Ok jsArray then
      // Convert to Baba Yaga list
      when (io.jsArrayToList jsArray) is
        Err msg then Err msg
        Ok babaList then
          // Process with Baba Yaga functions
          processed : map (x -> x * 2) babaList;
          filtered : filter (x -> x > 10) processed;
          
          // Convert back to JS array
          jsResult : io.listToJSArray filtered;
          
          // Return as JSON
          io.callJS "JSON.stringify" [jsResult];

// Usage
input : "[1, 2, 3, 4, 5, 6, 7, 8, 9, 10]";
result : processJSArray input;
// Returns: Ok (JSValue "[4,6,8,10,12,14,16,18,20]")
```

This JavaScript interop system provides a safe, controlled way to leverage JavaScript's ecosystem while maintaining Baba Yaga's functional programming principles and explicit error handling.