Installation

``

global

npm install -g tsst

local

npm install tsst --save-dev
`

What is it?

Uhh. This is an experimental library/tool/utility/thing to aid testing of TypeScript _semantics_.

What does it do?

At its core, this is a TypeScript transformer that takes semantic build errors and turns them into runtime errors. Here's the show-don't-tell explanation.

$3

`ts
type ZeroOneToBoolean
= {"0": false, "1": true}[T];

describe("ZeroOneToBoolean", () => {
it("works with '0'", () => {
type A = ZeroOneToBoolean<"0">;
});

it("works with '1'", () => {
type A = ZeroOneToBoolean<"1">;
});

// This test should fail for demonstration purposes
it("works with the number 0", () => {
type A = ZeroOneToBoolean<0>;
});
});
`


$3


`ts
describe("ZeroOneToBoolean", () => {
it("works with 0", () => {
});

it("works with 1", () => {
});

// This test should fail for demonstration purposes
it("works with the number 0", () => {
throw new Error("​​Type '0' does not satisfy the constraint '\"0\" | \"1\"'.​​");
});
});
`

In this early form, this only tests for successful compilation.

What might it do soon?

- [ ] Negative tests that let you expect specific errors
- [x] Have some form of cli that makes it more usable
- [ ] Have its own tests. This project is currently horrendously untested because I'm not sure what the best way to test this is.
- [ ] Have a better name?
- [ ] Change completely in any number of ways
- [ ] Stop parsing full tests and become a block-local transformation

How does one use this?

If you install this package globally, you can use tsst to transform the tests. This is a very basic builder. It uses the local tsconfig.json and takes a single argument for a test glob.

`


global install

tsst "*/.test.ts"

local install

node_modules/.bin/tsst "*/.test.ts"
`

The TypeScript compiler provides hooks for specifying custom transformers, but tsc does not expose these. While many common tools like ts-loader are starting to support transformers, most don't yet provide transformers with access to the ts.Program` object that this requires.

I've been messing about with Wallaby.js a bit recently, and tried to get it working with that, to partial success. They were absolutely fantastic about adding support for transformers for me with a feature-request-to-release turn-around time of about 2 hours. Wallaby.js runs and reports the transformed tests correctly, but there are some problems with line numbers and error positions not lining up. I haven't fully pinned these down yet, and am not sure to what degree these issues are solvable by this tool.