🔎 Search Terms

"array", "noUncheckedIndexedAccess", "5.5"

🕗 Version & Regression Information

• This changed between versions 5.4.5 and 5.5.2

⏯ Playground Link

https://www.typescriptlang.org/play/?noUncheckedIndexedAccess=true&noUnusedLocals=true&noUnusedParameters=true&ts=5.5.2#code/MYewdgzgLgBAhgLhmArgWwEYFMBOBtAXRgF4Y8BGAGhgCZqBmAgbgCgAbLWASyVU1xIxyrFlwBmACigBPAA5YQY+Hi5Fi6mACI+2HJoCULGMeWqA1GaA

💻 Code

const a: number[] = [1, 2, 3];
let i: number = 1;

if(typeof a[i] === "number")
    a[i]++

🙁 Actual behavior

Typescript reports the following error:

Object is possibly 'undefined'.(2532)
const a: number[]

🙂 Expected behavior

It should not report an error for such cases

Additional information about the issue

This obviously requires noUncheckedIndexedAccess to be enabled in the tsconfig.

This only happens when the variable used to access the array is non const (let in this case).

I think that TS 5.5 actually fixed the original behavior because in 5.4 if we remove the check:

const a: number[] = [1, 2, 3];
let i: number = 1;
a[i]++

TS does not raise the same error (which would be expected as a[i] could be undefined).
The current workaround:

const j = i;
if (typeof digits[j] === "number") {
  digits[j]++;
}