systemjs/docs/hooks.md

Loader Hooks

Hooking the Loader

The loader is designed to be hookable in a very light-weight way using only function extensions.

The standard pattern for this is:

const existingHook = System.constructor.prototype.hookName;
System.constructor.prototype.hookName = function (args) {
  return Promise.resolve(existingHook.call(this, args))
  .then(function (existingHookResult) {
    // custom hook here
    return ...;
  });
};

When hooking the loader it is important to pay attention to the order in which hooks will apply, and to keep existing hooks running where they provide necessary functionality.

In addition, some hooks are Promise-based, so Promise chaining also needs to be carefully applied only where necessary.

Core Hooks

createContext(url) -> Object

Used to populate the import.meta for a module, available at _context.meta in the System.register module format.

The default implementation is:

System.constructor.prototype.createContext = function (url) {
  return {
    url
  };
};

getRegister() -> [deps: String[], declare: Function]

This hook is intended for custom module format integrations only.

This function stores the last call to System.register, and is the companion hook for that function.

It is important that this function is synchronous, as any event loop delay would result in uncertainty over which source evaluation resulted in this registration call.

Custom module format support like AMD support is added by hooking the AMD registration in hook.

resolve(id, parentUrl) -> Promise

In the minimal s.js implementation, resolve is implemented as a synchronous function, so Promise.resolve should be used when extending this loader.

Resolve should return a fully-valid URL for specification compatibility, but this is not enforced.

onload(url, error) (sync)

This hook is not available in the s.js minimal loader build.

For tracing functionality this is called on completion or failure of each and every module loaded into the registry.

Such tracing can be used for analysis and to clear the loader registry using the System.delete(url) API to enable reloading and hot reloading workflows.

Extras Hooks

transform(url, source) -> Promise

This hook is provided by the transform extra.

The default implementation is a pass-through transform that returns the fetched source.

For an example of a transform see the Babel plugin transform.