Developing a JSX framework and jsxImportSource makes TypeScript slow

Bug Report

I created mini-jsx, a tiny library for creating native DOM nodes using JSX syntax, written in TypeScript.

Since I changed the code to define a classic runtime to an automatic runtime, TypeScript is 10x slower when run inside the project.

To try it, clone and setup the project:

$ git clone https://gitlab.com/appsemble/mini-jsx.git
$ cd mini-jsx
$ yarn

Run tsc and notice it takes a long time. Also opening it in VS Code will make the editor unresponsive.

$ yarn tsc  
yarn run v1.22.5
$ /home/remco/Projects/mini-jsx/node_modules/.bin/tsc
✨  Done in 11.56s.

Now switch to the branch before the automatic runtime was introduced, and notice everything works fine:

$ git checkout 19ded3c749fcbea5347b2cf3d30942ee0957ae41
$ yarn tsc
yarn run v1.22.5
$ /home/remco/Projects/mini-jsx/node_modules/.bin/tsc
✨  Done in 1.65s.

🔎 Search Terms

• jsxImportSource
• jsx import source
• jsx automatic runtime

🕗 Version & Regression Information

The issue has existed from the moment support for the automatic runtime was added in TypeScript 4.1, and still exists in version 4.2.x.

• I was unable to test this on prior versions because this feature didn’t exist then

⏯ Playground Link

The playground doesn’t offer the options needed to show the problem.

💻 Code

jsx-runtime.ts

export const jsx = <T extends keyof HTMLElementTagNameMap>(
  tag: T,
  { children, ref, ...props }: Props<T> = {}
): HTMLElementTagNameMap[T] => {
  const node = document.createElement(tag);
  const appendChildren = (child: Children): void => {
    if (Array.isArray(child)) {
      child.forEach(appendChildren);
    } else if (child != null && child !== true && child !== false) {
      node.append(child as Node);
    }
  };
  Object.entries(props).forEach(([key, value]) => {
    if (key in node) {
      type Key = keyof typeof node;
      if (node[key as Key] instanceof Object && value instanceof Object) {
        Object.assign(node[key as Key], value);
      } else {
        node[key as Key] = value as typeof node[Key];
      }
    } else {
      node.setAttribute(key, value as string);
    }
  });
  appendChildren(children);
  if (ref) {
    ref(node);
  }
  return node;
};

export const jsxs = jsx;

jsx-runtime.test.ts

import "./jsx-runtime";

it("assign properties", () => {
  const button = <button className="is-primary" type="button" />;
  expect(button).toBeInstanceOf(HTMLButtonElement);
  expect(button.className).toBe("is-primary");
});

🙁 Actual behavior

Running tsc takes 10x longer to run, VS Code becomed slow and unresponsive when working on the project, lag, pending code actions from ESLint, and TypeScript actions on save.

🙂 Expected behavior

Given this project is very small, running tsc takes about 1 second and VSCode is snappy.