Self Loading

ES6 defines a Loader object to load modules. Traceur is written in ES6. So how does traceur load itself?

Node

The command ./traceur runs node and uses node's loader (require()) to load src/node/command.js.

src/node/command.js processes the command line and loads src/node/traceur.js.

src/node/traceur.js reads the file bin/traceur.js and applies eval() to its content.

bin/traceur.js starts with some non-transcoded files from src/runtime, followed by lots of calls to ModuleStore.registerModule(). Each call is the output of compiling one module definition. Thus static module compile into calls to ModuleStore.registerModule().

At the very end of bin/traceur.js, a global variable traceur is set to the module stored as a result of compiling src/traceur.js. The access will trigger many module definition calls, that is calls to the functions stored when we called ModuleStore.registerModule(). This traceur global allows some access to traceur internals, including the Loader object.

Returning to src/node/command.js, we finish processing the command line then call one of three functions to compile a file, a directory, or to interpret a module. These functions use the traceur object to access the Loader.

In node, the System global is attached by require('./System') from src/node/System.js.

Web

In a Web page, we simply include bin/traceur.js which fills the ModuleStore as above and evaluates traceur at the end.

In a Web page, the System global is attached by loading the System module; this is done currently when traceur loads System module, but that prevents alternative system implementations.