Advanced

Three chapters on what TJS can do because its types and tests survive to runtime, things most languages leave to separate tools. They are written for readers comfortable with JavaScript; if you came through Learn to Program, finish it first. Each chapter stands alone, and all three assume the Basics.

  1. The Universal Endpoint builds one server endpoint that runs code sent by the client, in place of one route per operation, so the client's logic travels to the data. The request is an AJS program. AJS is a sandboxed subset of JavaScript that compiles to a JSON syntax tree and runs on a metered virtual machine, with a fuel limit (a work budget, so the program must stop) and only the capabilities you hand it (a store, fetch, an LLM: nothing else is reachable). This chapter assumes you have read The AJS Language, the first chapter of the AJS book.
  2. Inline Tests: Test Private Functions shows tests that live beside the code they test. They run when TJS converts the file to JavaScript and are left out of the JavaScript it produces, so they can reach functions the module never exports.
  3. Predicate Types shows a type defined by a function rather than by an example value (TJS's usual name: 'Alice' form): a CSS colour, say, which no example can describe. The same predicate can be mined for autocomplete suggestions. The compiler checks that the predicate is pure (no side effects) and fuel-bounded (it cannot hang), so it is safe to run on untrusted input. It is not a sandbox for untrusted source: a verified predicate compiles to native JavaScript, so run only predicates you trust. Untrusted code belongs in the AJS VM, as in the first chapter.

After this section you should know how TJS and AJS fit together to move code to the data instead of moving the data, how to test code that is not exported, and when to reach past examples for a predicate.