Slow compilation with nested optional chaining

[dlittleton]

🔎 Search Terms

"optional chaining", "slow compilation", "performance"

🕗 Version & Regression Information

• Reproducible on 5.7.3 and nightly Version 5.9.0-dev.20250225
• Trying different versions on the playground it appears that the behavior changed between 4.6.4 and 4.7.4

⏯ Playground Link

https://www.typescriptlang.org/play/?ts=5.9.0-dev.20250225#code/C4TwDgpgBAchDOwIBMoF4oG8BQU9QGMALASwBtkAnCAOwC5YElkBtAXQBpd8A3AQzIBXCAxqCAtgCMIlbAF9s2AgHsaiKPAh9KxKAziIU6LNzzFyVWg3Zd8UfkJFQArPMUq1wKH2ObtxADpzCmoaAH4AgDMSGmQASViIAA8AChSaAEp0AD4oGgCHYXQMAEYMgG4lVXVJXy0dIiDSENoWPjYI4MtwqJj4xNT0rLRc-MLoNAwAJgqqz0I6-0au0LaOpotVyXWV2gjo2ITkZLTMnLyCgSLJqABmWY91VAw-Bo2WmjXO5u6Wbe-Nq0CDsfqFeocBqdhqNLo5ilAACwPapeCYaeqBXafdoAj5-EGAz7A3G-ZAEj7g-rHQZnEYXcbw5zI+aRRZvLFfd6-f5c1bE3mtMkk1YQcndSlHE5Dc5jK4TDAANmZ6gA5mzMaDWjiBZ8eRz+RyhTqWKLha1ImKwQcqVLaTCGTcAOyzObwZRkCABMjKFUpADkABFVBA-RUgA

💻 Code

type Nested = {
    children: Nested[],
    value: number
}

const search : Nested = {
    children: [],
    value: 5
}

const a = search.children?.findIndex((n) => n.value == 1);
const b = search.children[a]?.children?.findIndex((n) => n.value == 2);
const c = search.children[a]?.children[b]?.children?.findIndex((n) => n.value == 3);
const d = search.children[a]?.children[b]?.children[c]?.children.findIndex((n) => n.value == 4);
const e = search.children[a]?.children[b]?.children[c]?.children[d]?.children.findIndex((n) => n.value == 5);
const f = search.children[a]?.children[b]?.children[c]?.children[d]?.children[e]?.children.findIndex((n) => n.value == 6);
const g = search.children[a]?.children[b]?.children[c]?.children[d]?.children[e]?.children[f]?.children.findIndex((n) => n.value == 7);

console.log('Done');

🙁 Actual behavior

Compilation time gets increasingly slow as more nested accesses using optional chaining are added.

Timing on my machine:

• 5 chains: < 1s
• 6 chains: 8s
• 7 chains: 220s

🙂 Expected behavior

Compilation time remains about the same or increases linearly.

Additional information about the issue

Originally found while diagnosing slow compile times in a project dealing with searches through a potentially nested API response.

Output of tsc --diagnostics:

Files: 58
Lines: 10638
Identifiers: 8659
Symbols: 7911
Types: 3124
Instantiations: 1150
Memory used: 56818K
I/O read: 0.02s
I/O write: 0.00s
Parse time: 0.12s
Bind time: 0.04s
Check time: 165.76s
Emit time: 56.28s
Total time: 222.20s

As a workaround, refactoring to assign the repeated index lookups to intermediate variables resolves the performance issue.