On the frontend, Submitty uses JavaScript to allow for interactivity with the site. Historically, the JS code was written and imported globally and that lives in the site/public/js folder. However, embracing modern standards, we are increasingly looking to write all new frontend code as ES6 modules.

ES6 Modules allow JavaScript code to be split up into “modules” that can allow other modules to import functions and variables. Modules in Submitty are loaded the same way as normal JavaScript files using the PHP function $this->output->addInternalJs(...). Unlike normal JavaScript, modules are “bundled” together by esbuild. This process takes all files imported and creates a single, minified JavaScript file that is served to the user.

Unlike regular JS files, ES6 modules are not included into the global JS scope, and are self-contained. When a module is executed, it only has access to the things that it imports, and we cannot call functions defined within modules directly from the DOM. As such, to hook a function defined in a module to something in the DOM, such as a click event listener, you will need to use the addEventListener API. This seperation of regular JS and modules also extends to HTML, attributes such as onclick and other inline event handlers cannot call functions defined in modules even if they are exported.

JavaScript modules are placed in the Submitty/site/ts directory and running the install script will launch esbuild and bundle files, the output will be placed in the Submitty/site/mjs directory. You can run the command npm run build to manually bundle files after running npm install from the Submitty/site directory.

npm run build uses node to run the Submitty/site/.build.js file, this contains the logic for searching which files to transpile and bundle, along with the configuration for esbuild.

Creating new modules

Any new module written should be a .ts file to take advantage the type safety of TypeScript. Top levels files under the ts directory are considered entry points and once bundled will create a corresponding .js file under the public/mjs directory along with a sourcemap. Support files for your module can be created under a new directory in the ts area, like the utils directory. On the PHP side, you will then only need to include the single corresponding mjs file to your top level entry point file. This has the advantage of reducing the number of files sent to the client.


TypeScript is a superset of JavaScript that allows strongly typed syntax which is then transpiled into normal JavaScript. Any file with the .ts file type will be transpiled by esbuild as part of the npm run build command and Submitty install script.

Loading JavaScript files

Once a module or regular JavaScript file is written it needs to be served to the user and included by the HTML. This is done in PHP through the addInternalModuleJs and addInternalJS. Each function takes a target filename which is then sent to the user and timestamped to prevent browsers from caching old versions. These functions are defined in the site/app/libraries/Output.php.

Both functions work similarly but the addInternalJS searches the site/public/js directory and addInternalModuleJs searches the site/public/mjs directory.


// searches the site/public/mjs area, its source file would be site/ts/foo.ts
// searches the site/public/js area