Slow typings related to large enums

[Spacetech]

Bug Report

🔎 Search Terms

Slow, enums, large. Related issue that looks similar: #42824

🕗 Version & Regression Information

Not sure when this started. I'm testing on 5.0.4

• This is a performance problem

⏯ Playground Link

Playground link with relevant code

💻 Code

The code is in the playground link. The exact tsconfig.json that I'm testing with is:

{
  "compilerOptions": {
    "target": "ES2022",
    "module": "commonjs",
    "declaration": true,
    "esModuleInterop": true,
    "forceConsistentCasingInFileNames": true,
    "strict": true,
    "skipLibCheck": true
  }
}

🙁 Actual behavior / 🙂 Expected behavior

The code in the playground link has an enum containing 753 values and contains the following ITest interface:

interface ITest {
    transforms?: PartialRecord<ItemType, ItemType>;
}

where PartialRecord is defined at the top of the file as:

export type PartialRecord<K extends number, V> = { [key in K]?: V };

Running a trace build / analyze results in:

PS Z:\Data\Downloads\test> tsc --generateTrace tstrace && npx analyze-trace tstrace --forceMillis 100
Hot Spots
└─ Check file z:\data\downloads\test\index.ts (259ms)
   └─ Check variable declaration from (line 777, char 11) to (line 777, char 61) (189ms)
      └─ Check expression from (line 777, char 20) to (line 777, char 61) (186ms)

The hotspot is line 777 - const target = items[itemType1]?.transforms?.[itemType2];

Now if I change the ITest interface to the following:

interface ITest {
    transforms?: { [key in ItemType]?: ItemType };
}

(I'm essentially using the same typing that PartialRecord<ItemType, ItemType> provides)

The performance is improved now, it's 2.5x faster. The self-time for the expression on line 777 went from 171ms to 66ms -

PS Z:\Data\Downloads\test> tsc --generateTrace tstrace && npx analyze-trace tstrace --forceMillis 100
Hot Spots
└─ Check file z:\data\downloads\test\index.ts (153ms)

My codebase has a lot of large enums, so this matters quite a bit to us in the overall scheme of things.

Is it possible to speed up the typing in the case when we use something like PartialRecord with large enums?

[ahejlsberg]

Hmm, yeah, there's some quadratic behavior going on here in our union type construction.

[fatcerberus]

It seems really weird that a type alias would be slower than using the exact same type it resolves to directly.