Modules with module-type: startup have to export a function named startup and may export a name property to identify this module. The startup function will be executed by the boot kernel at the end of the boot process.

// From boot.js:
// Gather up any startup modules
$tw.boot.remainingStartupModules = []; // Array of startup modules
$tw.modules.forEachModuleOfType("startup",function(title,module) {
	if(module.startup) {
		$tw.boot.remainingStartupModules.push(module);
	}
});
// Keep track of the startup tasks that have been executed
$tw.boot.executedStartupModules = Object.create(null);
$tw.boot.disabledStartupModules = $tw.boot.disabledStartupModules || [];
// Repeatedly execute the next eligible task
$tw.boot.executeNextStartupTask();

executeNextStartupTask() will execute the remaining startup modules in remainingStartupModules. A startup module can export the variables before and/or after, each containing an array of names of other startup modules. executeNextStartupTask() will use this information to execute the modules in the correct order.

Startup modules can be marked as synchronous by exporting synchronous = true. If synchronous is set to false, the startup function is executed with an callback function as the first argument. The startup function has to call this function to allow subsequent startup modules to get executed. This is necessary when the startup function itself uses asynchronous calls.