Tuple should be readonly by default

[Razdva122]

Any tuples can mutate, this entails incorrect typing

TypeScript Version: 4.1.0-dev.20200828

Search Terms:
"readonly tuple"
"immutable tuple"

Code

const exampleSort: [number, string] = [2, '1'];

exampleSort.sort();

exampleSort // Type: [number, string], RealType: [string, number]

const exampleSplice: [number, string] = [2, '1'];

exampleSplice.splice(0, 1);

exampleSplice // Type: [number, string], RealType: [string]

Expected behavior:

const exampleSort: [number, string] = [2, '1'];

// Property 'sort' does not exist on type '[number, string]'.(2339)
exampleSort.sort();

const exampleSplice: [number, string] = [2, '1'];

// Property 'splice' does not exist on type '[number, string]'.(2339)
exampleSplice.splice(0, 1);

Actual behavior:

const exampleSort: [number, string] = [2, '1'];

exampleSort.sort();

const exampleSplice: [number, string] = [2, '1'];

exampleSplice.splice(0, 1);

Playground Link: https://www.typescriptlang.org/play?ts=4.1.0-dev.20200828#code/MYewdgzgLgBApgDwIYFsAOAbOBlEAnKALhgG0wBXFAIzjwBoZo8BLMAcwF0YBeUgJgYByAIyCOAbgBQkxKkw58UAHQRFACgCUUmcnRZcBGAHojMACoBPNHGJlKNeoygt2HBgCU4SDJeu2mrGwMFNS0HNKgkLCyejiYzMA2pCEODAGuPPxCohLSMfLY8YkqRXBqAAwMwlp5ugWlxqa+SXahjumcHl4+Vi0d4dKSQA

Related Issues:

[jcalz]

Note that something like this was previously declined in #6325.

I don't think it should be readonly since you should be able to write into it the way you would with any object:

const x: [number, string] = [1, "hello"];
x[0]++; // should be fine
x[1] += " there"; // should be fine

Instead, you'd probably just want to omit methods that can violate tuple constraints like "pop" | "push" | "reverse" | "shift" | "sort" | "splice" | "unshift" | "fill" | "copyWithin".

(Aside: it's fun to think about which tuple types could still allow which methods safely... perhaps [string, string, string] could still allow sort() and [number, string, number] could still allow reverse() and [string, ...string[]] could still allow push() and unshift().)

---

But I imagine that any form of this would be a big breaking change, right? Lots of code has Foo[] in places where readonly Foo[] would be fine, and having tuples suddenly not assignable to array types would mess that up.

[RyanCavanaugh]

Merging in additional issue: This should apply to all mutating methods (splice, shift, unshift, etc)

[GermanJablo]

I love @jcalz's idea!

This could be implemented as a new configuration flag, just like what happened with --noUncheckedIndexedAccess to avoid a breaking change.

And to fix (I think) the problem @RyanCavanaugh mentions here, arrays could be made tuple-incompatible (i.e. a function that receives an array as a parameter should throw an error when you try to pass it a tuple).