Replace Node interface with discriminated unions

[JoshuaKGoldberg]

🔍 Search Terms

discrimnated type union node internal source api

✅ Viability Checklist

• This wouldn't be a breaking change in existing TypeScript/JavaScript code
• This wouldn't change the runtime behavior of existing JavaScript code
• This could be implemented without emitting different JS based on the types of the expressions
• This isn't a runtime feature (e.g. library functionality, non-ECMAScript syntax with JavaScript output, new syntax sugar for JS, etc.)
• This feature would agree with the rest of our Design Goals: https://github.com/Microsoft/TypeScript/wiki/TypeScript-Design-Goals

⭐ Suggestion

Happy Halloween 🎃! In the spirit of the holiday, I'd like to resurrect #13634: using a discriminated unions for Node categories. Roughly paraphrasing #18285's very good description here:

As mentioned in #18214 (comment), TypeScript's source can remove a lot of casts by using a discriminated union for Node.
While this should not affect TypeScript's runtime performance (as it just changes its types, mostly) it may cause TypeScript to take longer to compile itself.

#13634 was declined in 2018 for performance concerns (#13634 (comment)) but I hope that the years of performance improvements since then have since made this possible.

📃 Motivating Example

Switching TypeScript's Node from an interface from a discriminated type union would allow .kind checks to narrow types of values declared as Node.

import * as ts from "typescript";

declare const node: ts.Node;

if (node.kind === ts.SyntaxKind.FunctionDeclaration) {
  node.name?.text;
  //   ~~~~
  // Property 'name' does not exist on type 'Node'.
}

💻 Use Cases

In TypeScript itself, a match-case-and-full-word search for /(?:name|node|parent) as \w{5,}/ in src/**/*.ts excluding *test* yields 1879 matches right now. After prototyping it locally and enabling @typescript-eslint/no-unnecessary-type-assertion, I mostly-automatically reduced it to 987 matches. More removals could likely be enacted with manual effort.

• Previous attempt: Use discriminated unions for node categories #18285
• My attempt: [Reference] feat: switched Node to a discriminated union #56274
• (edit) Wes' PR, which everyone else in this thread except Andarist has previous commented on but somehow forgot about: Use union types for all common classes of AST nodes #54148

In community projects that use the TypeScript AST, including typescript-eslint, we'd similarly be able to have more precise type information on TS AST nodes.

#13634 mentions other consumers too, such as Babel's https://github.com/babel/babel/blob/c446ff85c28e117ebf3cd72cd34ef358f1077aa8/packages/babel-types/src/validators/is.ts.

[jakebailey]

@weswigham I know you were working on this somewhat recently too

[Andarist]

For reference: Wes' PR can be found here

[weswigham]

Yeah, I have a recentish branch there, the 2.7x slowdown in compilation time and editor performance hurts though. Personally, I'm of the opinion that we should just eat the difference and use the worsened conditions as a forcing function for self-improvement, but that order of operations isn't a popular one :P

So... we probably can't take the change until we get the perf closer, which means looking into those traces @jakebailey took and trying to optimize those cases.

[jakebailey]

It really goes to show how bad my memory is to not remember that there already was a PR, and also I reviewed it.