denoland-deno/deno_typescript

Deno TypeScript Crate

This crate provides utilities to compile typescript, bundle it up, and create a V8 snapshot, all during build. Snapshots allow the executable to startup fast.

system_loader.js

This is a minimalistic implementation of a System module loader. It is specifically designed to load modules that are emitted from TypeScript the module format is "system" and a single "outfile" is supplied, which is commonly refereed to as a bundle.

Because this loader becomes part of an emitted bundle under Deno.bundle() and deno bundle, it has minimal comments and very terse and cryptic syntax, which isn't very self documenting. Because of this, a guide to this file is provided here.

A bundle of System modules expects a System.register() function to be in scope for registering the modules. Modules that are emitted from TypeScript in a single out file always pass 3 arguments, the module specifier, an array of strings of modules specifiers that this module depends upon, and finally a module factory.

The module factory requires two arguments to be passed, a function for exporting values and a context object. We have to bind to some information in the environment to provide these, so gC gets the context and gE gets the export function to be passed to a factory. The context contains information like the module specifier, a reference to the dynamic import() and the equivalent of import.meta. The export function takes either two arguments of an named export and its value, or an object record of keys of the named exports and the values of the exports.

Currently, TypeScript does not re-write dynamic imports which resolve to static strings (see microsoft/TypeScript#37429), which means the import specifier for a dynamic import which has been incorporated in the bundle does not automatically match a module specifier that has been registered in the bundle. The di() function provides the capability to try to identify relative import specifiers and resolve them to a specifier inside the bundle. If it does this, it resolves with the exports of the module, otherwise it simply passes the module specifier to import() and returns the resulting promise.

The running of the factories is handled by rF(). When the factory is run, it returns an object with two keys, execute and setters. execute is a function which finalises that instantiation of the module, and setters which is an array of functions that sets the value of the exports of the dependent module.

The gExp() and gExpA() are the recursive functions which returns the exports of a given module. It will determine if the module has been fully initialized, and if not, it will gather the exports of the dependencies, set those exports in the module via the setters and run the modules execute(). It will then always return or resolve with the exports of the module.

As of TypeScript 3.8, top level await is supported when emitting ES or System modules. When Deno creates a module bundle, it creates a valid, self-contained ES module which exports the exports of the "main" module that was used when the bundle was created. If a module in the bundle requires top-level-await, then the execute() function is emitted as an async function, returning a promise. This means that in order to export the values of the main module, the instantiation needs to utilise top-level-await as well.

At the time of this writing, while V8 and other JavaScript engines have implemented top-level-await, no browsers have it implemented, meaning that most browsers could not consume modules that require top-level-await.

In order to allow more browsers to consume bundles, there is an argument that is passed to the __instantiate() function which determines if the code is bootstrapped asynchronously or not. When emitting a bundle that contains a module that requires top-level-await, Deno will detect this and utilise await __instantiate(main, true).

The system_loader_es5.js is a transpiled version of system_loader.js that is designed to work with ES5 or later, and will be used when the bundle target is < ES2017. While ES3 is still a potential target which can be passed in a tsconfig.json to Deno, any resulting bundle will not be compatible, as there is a need to utilise items like Object.defineProperty().