Readonly spread array in tuple-CodePudding

I have some TypeScript code that I mean not to be executed, but it does, why?

let a:[boolean, ... readonly string[]] = [true, '1', '2', '3'];

a[0] = false;
a[1] = 'one'; //no error, but it should be error

CodePudding user response：

The reason your code doesn't produce an error is that the type of a is immediately reduced by the compiler to the fully-mutable type [boolean, ...string[]], despite your annotation:

let a: [boolean, ... readonly string[]] = [true, '1', '2', '3'];

type A = typeof a;
// type A = [boolean, ...string[]]

And therefore none of the elements of a are read-only. So that's the answer to the question as asked.

The obvious follow-up question is "why does it get reduced to a fully mutable type?". Unfortunately I can't find any authoritative source for this, so anything I could say here would be speculation. Here it comes:

The type [T, ...A] is the type you get when you spread an array of type A into the end of an array literal that starts with a value of type t. The fact that the following function compiles with no error is evidence of this:

function f<T, A extends readonly any[]>(t: T, a: A): [T, ...A] {
    return [t, ...a]
}

And if I have a read-only string array and spread it to the end of an array literal, the resulting array will not be read-only:

const strs: readonly string[] = ["x", "y"]
const result = f(true, strs);
// const result: [boolean, ...string[]]

So that's my guess. If anyone finds any canonical source (e.g., TS doc or GitHub issue) that confirms or refutes this, I'd be interested to know about it.

Playground link to code

CodePudding user response：

Spreading an array/tuple just orderly lists out the element of that array.

It just plucks out the values from the array without actually modifying/transferring the inferred characteristics

let xs = [1,2,3,4]

let ys = [...xs] // [1,2,3,4]

Hence when we do

let a:[boolean, ... readonly string[]] = [true, '1', '2', '3'];

We are just saying pluck all the values from the readonly string array and not that make the values as readonly.

Thus the behavior.

CodePudding user response：

It works because readonly is only the individual elements themselves. The array itself is still mutable.

If you did

let a:readonly [boolean, ... readonly string[]] = [true, '1', '2', '3'];

a[0] = false; // error
a[1] = 'one'; //error

then, yes, it errors because you've marked the array itself as readonly, not just some of its values.