Array: Object is possible 'undefined'.

[infacto]

Bug Report

🔎 Search Terms

array is possible undefined

🕗 Version & Regression Information

Tested with typescript@next (4.6.0-dev.20211216). I initially came from 4.2.4.

⏯ Playground Link

Playground does not work for this.

💻 Code

export const myFunc = (): void => {
  const bytes: number[] = new Array<number>(16).fill(0);
  bytes[0] = bytes[6] & 0xf0;
};

🙁 Actual behavior

I get an error Object is possiible 'undefined'.

🙂 Expected behavior

I know that new Array(n) generates an array with length and undefined items. But I also use fill and fixed types. I don't know how to avoid this error. Cast to number[] does also not work (unsafe cast).

tsconfig.json (click to expand)

{
  "compilerOptions": {
    "strict": true,
    "noImplicitReturns": true,
    "noFallthroughCasesInSwitch": true,
    "noUncheckedIndexedAccess": true,
    "target": "ES2018",
    "module": "commonjs",
    "moduleResolution": "Node",
    "importHelpers": true,
    "declaration": true,
    "declarationMap": true,
    "sourceMap": true,
    "rootDir": "src",
    "incremental": true,
    "tsBuildInfoFile": "node_modules/.cache/.tsbuildinfo",
    "noErrorTruncation": true
  },
  "include": ["src"],
  "exclude": ["./**/*.spec.ts", "tests"]
}

[MartinJohns]

Playground does not work for this.

It does, you just need to enable noUncheckedIndexAccess: Playground link

And then it's basically the same as #46922. The type number[] has no notion of "index 6 is always defined".

[infacto]

Ok thanks. Disabling noUncheckedIndexedAccess works.
But are there other options? I mean the array declaration is safe without undefined items withing the given length (in this case). Can I force the not nullish type like with ! or something like that?

[fatcerberus]

Sure, if you know for sure the value won’t be undefined then just write bytes[6]!.