immutable string

[danielnixon]

Strings are Immutable at runtime, but only (strictly speaking) ReadonlyDeep at compile time.

As so often, the cause is method syntax, in particular:

interface RelativeIndexable<T> {
    /**
     * Takes an integer value and returns the item at that index,
     * allowing for positive and negative integers.
     * Negative integers count back from the last item in the array.
     */
    at(index: number): T | undefined;
}

Observe:

const foo = "hello";

// This compiles but throws at runtime
// eslint-disable-next-line functional/no-expression-statements, functional/immutable-data, functional/functional-parameters
foo.at = () => "";

It would be nice (if extremely low priority) if this could be caught at compile time, before linting and certainly before runtime.

[danielnixon]

This trick fixes it:

export type ImmutableString = ImmutableShallow<string>;

const foo: ImmutableString = "";

// No longer compiles: Cannot assign to 'at' because it is a read-only property. ts(2540)
// eslint-disable-next-line functional/no-expression-statements, functional/immutable-data, functional/functional-parameters
foo.at = () => "";

[danielnixon]

ImmutableString would be only a novelty in practice, because:

const a: string = "";
const b: ImmutableString = a;
// Type 'ImmutableString' is not assignable to type 'string'. ts(2322)
const c: string = b;