add array-map for keeping tuple length when using .map

[tjenkinson]

When you're using Array.map with a tuple, the length is lost. This means you loose the guard against accessing an item out of bounds.

// BEFORE
const tuple = [1, 2, 3] as const;
const mapped = tuple.map((a) => a + 1);
// oops. There's no 4th element, but no error
console.log(mapped[3]);

With array-map enabled, this code will now error:

// AFTER
import "@total-typescript/ts-reset/array-map";
const tuple = [1, 2, 3] as const;
const mapped = tuple.map((a) => a + 1);
// Tuple type 'readonly [number, number, number]' of length '3' has no element at index '3'.
console.log(mapped[3]);

[Eshanatnight]

// BEFORE
const tuple = [1, 2, 3] as const;
const mapped = tuple.map((a) => a + 1);
// oops. There's no 4th element, but no error
console.log(tuple[3]);

// AFTER
import "@total-typescript/ts-reset/array-map";
const tuple = [1, 2, 3] as const;
const mapped = tuple.map((a) => a + 1);
// Tuple type 'readonly [number, number, number]' of length '3' has no element at index '3'.
console.log(tuple[3]);

shouldnt the console.log be console.log(mapped[3]);

[tjenkinson]

Oops. Yep good spot thanks fixed now

[Eshanatnight]

It was just me being pedantic. 🤗

[ahrjarrett]

Suggested change

	): { [K in keyof this]: U };
	): { readonly [K in keyof this]: U };

[tjenkinson]

hey @ahrjarrett what difference does this make? Could we test it?

[ahrjarrett]

In my mind, if this doesn't seem to make a difference, that only happens to be true for the set of inputs we've tried it with.

Fixing the output of a readonly array's map operation to be readonly seems like a reasonable thing to do. It creates a cohesive API, its semantics are unambiguous, and it eliminates the possibility that one of the property modifiers might change out from under us.

Probably a moot point anyway, since I don't know if this addition is in keeping with the design goals of ts-reset. That said, I'm personally all for preserving structure like this where possible, so I went ahead and made the suggestion :)