Convert htmx to TypeScript 😍

TypeScript would bring lots of DX benefits to the htmx source code. Besides the obvious strict type checking, we would be able to write code using the latest and greatest ECMAScript 2021, but continue compiling to ES5 (to support IE11), or any version of ECMAScript really, with a single config setting.

{
  "compilerOptions": {
    "target": "ES5"
  }
}

I’ve started a WIP branch in which I’ve converted htmx.js to htmx.ts, changing the minimum amount of code possible, and it is compiling with all tests passing.

@1cg in theory, we could merge this branch into the dev branch before the next minor release, and start converting the source code incrementally.

[benpate]

I think this may have already been settled, but I’ll just add this for posterity. Typescript is made for codebases EXACTLY like htmx. The compiler can be set to work with existing *.js files, and there’s even an option to include (almost) all of the Typescript annotations as backwards-compatible JSDoc comments.

A small band of Typescript fanatics (including me) have been sneaking JSDoc/Typescript comments into htmx and hyperscript for a while, now 😃 This gives us the benefits of using Typescript with little additional developer overhead. The comments are a little more clunky than pure Typescript, but it lets us get started while we work out the infrastructure.

[gnat]

Of course up to @1cg to decide if this switch is actually worthwhile, but will have to say no thanks because:

• Increased barrier for contributions.
• More complex, slower build process.
• Type checking may not even be of tangible benefit for htmx (htmx has a nice test suite).

The ECMAScript spec will probably supersede TypeScript in the near future anyway. This has happened historically with other dead Javascript dialects (example: when classes were included in ES6).