Replace Node interface with discriminated unions

🔍 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 https://github.com/Microsoft/TypeScript/pull/18214#issuecomment-326823448, 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 (https://github.com/microsoft/TypeScript/issues/13634#issuecomment-413289616) 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: https://github.com/microsoft/TypeScript/pull/18285
• My attempt: https://github.com/Microsoft/TypeScript/pull/56274
• (edit) Wes' PR, which everyone else in this thread except Andarist has previous commented on but I somehow forgot about: #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.

Start by reviewing the earlier attempts in #18285, #56274, and #54148, then inspect the 1879 cast matches in src/**/*.ts mentioned in the issue. Done would mean replacing Node's interface with discriminated unions, removing justified casts where narrowing makes them unnecessary, and preserving runtime behavior without emitting different JavaScript.