Different behavior between const and readonly for multidimensional array access/strict null checking

[delosford]

🔎 Search Terms

different behavior between readonly and const

🕗 Version & Regression Information

• I was unable to test this on prior versions because I'm using TS Playground
• update: I tested it back a few versions after I was told how to on TS Playground and it produces the same errors.

⏯ Playground Link

No response

💻 Code

//this gives an error about possible null values
let someArray: Array<Array<Array<number | null> | null> | null> = new Array<Array<Array<number | null> | null> | null>();
let someInt = 0;

if (someArray[someInt] !== null && someArray[someInt][someInt] != null)
 console.log(someInt);

//this gives no error about possible null values, meaning the problem seems to be that someInt could theoretically be modified
let someArray: Array<Array<Array<number | null> | null> | null> = new Array<Array<Array<number | null> | null> | null>();
const someInt = 0;

if (someArray[someInt] !== null && someArray[someInt][someInt] != null)
 console.log(someInt);

//this however gives an error, with or without readonly

function itsaFunction(arrayAccessors: readonly number[])
{
	let someArray: Array<Array<Array<number | null> | null> | null> = new Array<Array<Array<number | null> | null> | null>();

	if (someArray[arrayAccessors[0]] !== null && someArray[arrayAccessors[0]][arrayAccessors[1]] != null)
 		console.log(arrayAccessors);
}

🙁 Actual behavior

Errors about possible null values do not treat const and readonly the same

🙂 Expected behavior

const and readonly should produce/fix the same errors in the same way

Additional information about the issue

No response

[MartinJohns]

• I was unable to test this on prior versions because I'm using TS Playground

Especially on the playground it's easy to test this on prior versions because the playground has a version selector on the top left.

[delosford]

• I was unable to test this on prior versions because I'm using TS Playground

Especially on the playground it's easy to test this on prior versions because the playground has a version selector on the top left.

I was unaware of that. Thanks for letting me know. I tested it back a few versions and they all produce the same errors.

[ahejlsberg]

This is working as intended. A readonly reference doesn't guarantee that the underlying object isn't mutated. I just guarantees that it isn't mutated through that reference. You need to store the value of arrayAccessors[0] in a const local to get the desired behavior.

[delosford]

function itsaFunction(arrayAccessors: readonly number[])
{
	let someArray: Array<Array<Array<number | null> | null> | null> = new Array<Array<Array<number | null> | null> | null>();

	const arrAcc1 = arrayAccessors[0];
	const arrAcc2 = arrayAccessors[1];

	if (someArray[arrAcc1] !== null && someArray[arrAcc1][arrAcc2] != null)
 		console.log(arrayAccessors);
}

Strangely, this doesn't work, either. Gives the same error.

[whzx5byb]

Strangely, this doesn't work, either.

This works in 5.5 beta. For more information, see https://devblogs.microsoft.com/typescript/announcing-typescript-5-5-beta/#control-flow-narrowing-for-constant-indexed-accesses

[delosford]

Strangely, this doesn't work, either.

This works in 5.5 beta. For more information, see https://devblogs.microsoft.com/typescript/announcing-typescript-5-5-beta/#control-flow-narrowing-for-constant-indexed-accesses

Thanks for the info. Good to know this is at least being addressed.

[typescript-bot]

This issue has been marked as "Working as Intended" and has seen no recent activity. It has been automatically closed for house-keeping purposes.