`noUncheckedIndexedAccess`: not working as expected in "for...of" loops

[Znerole]

Bug Report

🔎 Search Terms

• noUncheckedIndexedAccess
• for...of

🕗 Version & Regression Information

• typescript@4.1.2
• typescript@4.2.0-dev.20210203

⏯ Playground Link

Playground Link

💻 Code

// this example requires "noUncheckedIndexedAccess" to be enabled to make sense

function someNumberFunction(_: number) {}

const myArray: number[] = []
myArray[5] = 5

for(let i = 0; i < myArray.length; ++i) {
 const v = myArray[i] // v is "number | undefined"...
 if (v != null) { // ... so it needs to be checked for undefined
 someNumberFunction(v)
 }
}

for(const v of myArray) {
 // v is "number" according to TypeScript...
 // ... so we can erroneously call the function without checking for undefined
 someNumberFunction(v)
}

🙁 Actual behavior

With noUncheckedIndexedAccess enabled, value v in the for...of loop is still treated as number, allowing to unwittingly further process an undefined value.

🙂 Expected behavior

Value v should be treated as number | undefined to be consistent with index access on arrays.

[RyanCavanaugh]

"Correctly" handling sparse arrays are not part of noUncheckedIndexedAccess. See the PR notes - #39560

[Znerole]

Thank you, I overlooked that part and I get the rationale behind it. I wish JavaScript would differentiate between sparse and dense arrays, but here we are!

[RyanCavanaugh]

Me too 😅