typescript-diagnostic-tests

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.

Your tests

type ZeroOneToBoolean<T extends "0" | "1">
    = {"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>;
    });
});

What your tests become

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?

Good question, it's pretty tricky right now. 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.

Right now, the best way to use this is probably with a small custom TypeScript compiler script, I may include a generic solution soon.