Script files

For those more familiar with JavaScript coding, script files provide a code editor to write a JavaScript file which works independently of the event sheet.

Adding script files

Script files can be added in the Scripts folder of the Project Bar. Existing JavaScript files (.js) can also be imported using the Import scripts option instead.

Once you add a script a code editor appears. The first script added will have some default code added to help you get started.

Execution of the main script

Construct loads only the main script just after the Construct engine scripts run on startup. This is before any loading screen appears. It is a good place to write any initialization code and imports/exports the entire project will use. Since the Construct engine has still not yet initialised at this point, there is no runtime variable (representing the runtime script interface) available at the top level. Instead there is a special runOnStartup function that runs a callback once the runtime is ready, and provides the runtime variable as a parameter.

runOnStartup(async runtime =>
	// This code runs on startup with
	// the 'runtime' variable available

The callback can be an async function, meaning the await keyword can be used inside it. The top-level scope of a script is not allowed to use await, so this is a convenient place to run any asynchronous initialization, while also making use of the runtime script interface. All callbacks passed to runOnStartup are executed simultaneously just as the loading screen appears, and the runtime will wait for all the async functions to complete before starting the game.

Continuing execution

The only code in the main script that is automatically executed is the top-level scope and any runOnStartup callbacks. Beyond that no code in your script will run any more, unless you add event listeners to run callbacks. The main event to listen for is the runtime "tick" event. Since this fires every tick it provides a good place to keep running JavaScript code throughout your game. The code example below demonstrates a typical way to use this.

runOnStartup(async runtime =>
	runtime.addEventListener("tick", () => Tick(runtime));

function Tick(runtime)
	// Code to run every tick.
	// Note 'runtime' is passed.

Using additional scripts

As noted previously, the only script Construct automatically loads and runs is the main script. This appears in bold in the Project Bar.

When you select script files, the Properties Bar shows a Purpose property for the script. The main script has the purpose set to Main script, and you can only have one main script in your project.

To use any other script files in your project, you must import them from the main script. This also lets you control the order they are loaded and run. These other script files should have the Purpose set to '(none)' (indicating Construct won't use it automatically) and also export the things it wants to be used by other scripts.

To learn more about imports and exports, refer to the MDN guide on JavaScript Modules.

Integration with scripts in events

Unlike legacy "classic" mode scripts, modules have their own top-level scope. This means things like a top-level function declaration is not available in other script files.

Instead you can add imports to the script file with the purpose Imports for events which then become available for scripts in events. See the section Using imports in Scripts in event sheets for more details.

Another option is to write globals as explicit properties of globalThis, e.g. globalThis.myFunction = function () { ... } and call it via globalThis.myFunction(), but using modules is preferable.


Unlike scripts in event sheets, errors arising from the top level of script files are not automatically handled by Construct. If an unhandled exception is thrown, the browser will halt any further execution of script in that file. Typically this causes the rest of your code to stop working, and is considered a crash. See the section debugging scripts to find out how to deal with such issues.

Note one difference is exceptions or rejections in a runOnStartup callback are automatically handled by Construct. The error will be logged to the browser console and the runtime will continue to start up and run the game - but note if an error occurred it may not run as expected.

Construct 3 Manual 2021-02-03