JavaScript API

SkyScript injects the window.skyscript object into every page. The xplm namespace provides X-Plane SDK access, postMessage/onMessage enable bidirectional plugin communication, and the fs namespace offers scoped file system access. All async methods return Promises and communicate with the native plugin on the simulator's main thread.

Availability

The API is injected after each page load. It is safe to use once DOMContentLoaded has fired. You can check for its presence:

if (window.skyscript?.xplm) {
  // API is available
}

API Reference

X-Plane SDK (skyscript.xplm)

getDataref(ref)

Read the current value of an X-Plane dataref.

skyscript.xplm.getDataref(ref: string): Promise<number | string | number[]>

Parameters:

Name	Type	Description
ref	string	Full dataref path (e.g. "sim/time/zulu_time_sec")

Returns: A Promise that resolves with the dataref value. The return type depends on the dataref:

Dataref Type	JS Type	Example
int	number	1
float	number	42345.128906
double	number	42345.12890625
string / data	string	"KSFO"
float[]	number[]	[0.5, 1.0, 0.8]
int[]	number[]	[1, 0, 1, 1]

Example:

const zulu = await skyscript.xplm.getDataref('sim/time/zulu_time_sec');
console.log(`Zulu time: ${zulu}`);

const lat = await skyscript.xplm.getDataref('sim/flightmodel/position/latitude');
const lon = await skyscript.xplm.getDataref('sim/flightmodel/position/longitude');
console.log(`Position: ${lat}, ${lon}`);

Errors: The Promise rejects if the dataref is not found or has an unsupported type.

---

setDataref(ref, value, valueType?)

Write a value to an X-Plane dataref.

skyscript.xplm.setDataref(
  ref: string,
  value: number | string,
  valueType?: 'int' | 'float' | 'string'
): Promise<void>

Parameters:

Name	Type	Description
ref	string	Full dataref path
value	number | string	The value to set
valueType	string	(optional) Force a type. If omitted, inferred from value: integers map to "int", other numbers to "float", strings to "string".

Example:

// Set a float dataref
await skyscript.xplm.setDataref('sim/time/zulu_time_sec', 3600);

// Explicitly specify type
await skyscript.xplm.setDataref('sim/cockpit/radios/com1_freq_hz', 12180, 'int');

---

executeCommand(command)

Execute an X-Plane command (equivalent to XPLMCommandOnce).

skyscript.xplm.executeCommand(command: string): Promise<void>

Parameters:

Name	Type	Description
command	string	Full command path (e.g. "sim/operation/pause_toggle")

Example:

await skyscript.xplm.executeCommand('sim/operation/pause_toggle');
await skyscript.xplm.executeCommand('sim/GPS/g1000n1_hdg_sync');

Errors: Does not reject if the command is not found — X-Plane silently ignores unknown commands.

---

Message Passing (skyscript.postMessage / skyscript.onMessage)

SkyScript provides a bidirectional message channel between JavaScript and the native plugin (Go/C++). This enables plugins to expose custom functions and push data to the JS frontend without SkyScript needing to know the plugin's domain.

postMessage(channel, payload)

Send a message from JavaScript to the native plugin and wait for a response.

skyscript.postMessage(channel: string, payload: any): Promise<any>

Parameters:

Name	Type	Description
channel	string	Channel name identifying the handler (e.g. "getProfile")
payload	any	Data to send. Will be JSON-serialized automatically.

Returns: A Promise that resolves with the handler's JSON response, or rejects if the handler returns an error or no handler is registered for the channel.

Example:

// Request data from the plugin
const profile = await skyscript.postMessage('getProfile', { name: 'default' });
console.log(profile);

// Send data to the plugin
await skyscript.postMessage('saveProfile', { name: 'custom', buttons: [1, 2, 3] });

---

onMessage(channel, callback)

Register a listener for messages pushed from the native plugin to JavaScript.

skyscript.onMessage(channel: string, callback: (payload: any) => void): void

Parameters:

Name	Type	Description
channel	string	Channel name to listen on
callback	function	Called with the parsed JSON payload when the plugin sends a message

Example:

// Listen for updates from the plugin
skyscript.onMessage('profileUpdated', (data) => {
  console.log('Profile updated:', data);
  updateUI(data);
});

skyscript.onMessage('validationResult', (result) => {
  if (result.errors.length > 0) {
    showErrors(result.errors);
  }
});

Multiple listeners can be registered on the same channel. They are called in registration order.

---

File System (skyscript.fs)

The fs namespace provides scoped file system access. All paths are restricted to the plugin directory for security — attempts to access paths outside the plugin directory will be rejected.

readFile(path)

Read the contents of a file as a string.

skyscript.fs.readFile(path: string): Promise<string>

Parameters:

Name	Type	Description
path	string	Absolute path to the file (must be within the plugin directory)

Example:

const content = await skyscript.fs.readFile('/path/to/plugin/config.yaml');
console.log(content);

Errors: Rejects if the file is not found, cannot be read, or the path is outside the plugin directory.

---

writeFile(path, content)

Write a string to a file. Creates the file and any parent directories if they don't exist. Overwrites existing files.

skyscript.fs.writeFile(path: string, content: string): Promise<void>

Parameters:

Name	Type	Description
path	string	Absolute path to the file (must be within the plugin directory)
content	string	Content to write

Example:

await skyscript.fs.writeFile('/path/to/plugin/config.yaml', yamlContent);

Errors: Rejects if the file cannot be written or the path is outside the plugin directory.

---

listDir(path)

List the names of entries in a directory.

skyscript.fs.listDir(path: string): Promise<string[]>

Parameters:

Name	Type	Description
path	string	Absolute path to the directory (must be within the plugin directory)

Returns: A Promise that resolves with an array of file and directory names (not full paths).

Example:

const entries = await skyscript.fs.listDir('/path/to/plugin/profiles');
console.log(entries); // ["default.yaml", "custom.yaml"]

Errors: Rejects if the directory is not found or the path is outside the plugin directory.

---

exists(path)

Check whether a file or directory exists.

skyscript.fs.exists(path: string): Promise<boolean>

Parameters:

Name	Type	Description
path	string	Absolute path to check (must be within the plugin directory)

Returns: A Promise that resolves with true if the path exists, false otherwise.

Example:

if (await skyscript.fs.exists('/path/to/plugin/profiles/custom.yaml')) {
  const content = await skyscript.fs.readFile('/path/to/plugin/profiles/custom.yaml');
}

Errors: Rejects if the path is outside the plugin directory.

---

Console Logging

All console.log(), console.warn(), and console.error() calls from your app are forwarded to X-Plane's developer console (Log.txt). Messages are prefixed with the app name:

[SkyScript] [My App] [LOG] Hello from my app (http://localhost/index.js:42)
[SkyScript] [My App] [ERROR] Something went wrong (http://localhost/index.js:55)

This makes it easy to debug your app without needing browser DevTools.

---

How It Works

Understanding the communication flow helps when debugging:

1. Your JavaScript calls a method like getDataref("sim/time/zulu_time_sec").
2. The method creates a Promise and stores its resolve/reject callbacks with a unique ID.
3. A hidden <iframe> navigates to skyscript://xplm/getDataref?ref=sim/time/zulu_time_sec&callbackId=1.
4. The native plugin intercepts this URL in the CEF resource handler (IO thread) and enqueues the request.
5. On X-Plane's main thread (flight loop), the plugin dequeues the request and executes the XPLM SDK call.
6. The result is sent back by calling ExecuteJavaScript("window.skyscript._resolve(1, 42345.128906)").
7. Your Promise resolves with the value.

All XPLM calls happen on X-Plane's main thread, so there are no threading issues.

TypeScript Declarations

Add these to a .d.ts file in your project for full type safety:

interface SkyscriptXplm {
  getDataref(ref: string): Promise<number | string | number[]>;
  setDataref(ref: string, value: number | string, valueType?: string): Promise<void>;
  executeCommand(command: string): Promise<void>;
}

interface SkyscriptFs {
  readFile(path: string): Promise<string>;
  writeFile(path: string, content: string): Promise<void>;
  listDir(path: string): Promise<string[]>;
  exists(path: string): Promise<boolean>;
}

interface Window {
  skyscript: {
    version: string;
    xplaneVersion: string;
    xplm: SkyscriptXplm;
    postMessage(channel: string, payload: any): Promise<any>;
    onMessage(channel: string, callback: (payload: any) => void): void;
    fs: SkyscriptFs;
  };
}

Common Datarefs

Here are some frequently used datarefs to get started:

Dataref	Type	Description
sim/time/zulu_time_sec	float	Current Zulu time in seconds
sim/flightmodel/position/latitude	float	Aircraft latitude
sim/flightmodel/position/longitude	float	Aircraft longitude
sim/flightmodel/position/elevation	float	Altitude MSL (meters)
sim/flightmodel/position/y_agl	float	Altitude AGL (meters)
sim/flightmodel/position/groundspeed	float	Ground speed (m/s)
sim/flightmodel/position/indicated_airspeed	float	IAS (knots)
sim/flightmodel/position/mag_psi	float	Magnetic heading
sim/weather/wind_direction_degt	float	Wind direction (true)
sim/weather/wind_speed_kt	float	Wind speed (knots)

For a complete list, see the X-Plane DataRef list.