microsoft / microsoft/TypeScript

Fast path opportunity in `checkVariableLikeDeclaration`

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#53,108 2 comments 0 reactions 0 assignees View on GitHub

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Experience Enhancement Suggestion
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Description

I noticed that for this code:

class A {
  state = { foo: "foo", bar: 42 };
}

checkVariableLikeDeclaration calls checkTypeAssignableToAndOptionallyElaborate here:
https://github.dev/microsoft/TypeScript/blob/e9868e96e87996df46a13b4323866acc639e71ce/src/compiler/checker.ts#L40690

This is likely redundant for cases without a declared type as it's guaranteed that this has to return true. I thought at first that this would rely on a fast past based on the type.id but it seems that those types have different ids (perhaps one is fresh while the other one isn't or something?).

I'm happy to explore this optimization. But perhaps you'd have some preferences as to at which level this should be applied?

cc @jakebailey

Contributor guide

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First steps

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  3. Fork the repository and make your change on a branch.
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Research direction

Start in src/compiler/checker.ts at checkVariableLikeDeclaration and its call to checkTypeAssignableToAndOptionallyElaborate. Use the class-field example from the issue to investigate whether the check is redundant when no declared type is present and whether the differing type IDs matter. Confirm the optimization's behavior and measure whether it improves this path.

Written by the indexing model from the issue text.

Assessment

Tech stack
typescript
Domain
compilers, performance
Issue type
Refactor
Difficulty
4/5
Estimated time
3-5 days
Activity status
Stale
Clarity
Mostly clear
Newbie friendliness
35/100

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