To achieve this goal, one can utilize a "smart" setter approach. The concept involves defining a setter within the class, removing the setter from within the class itself, and creating a read-only property using the Object.defineProperty method with a descriptor where the writable property is set to false.
The aspect of being "changed within the class itself" can be accomplished through redefining another property. It is important to ensure that the property is initially made configurable.
Below is an example of how this process could look (there might not be a more elegant solution) - note that this runtime version of the answer is not specific to TypeScript:
class ReadonlyFields {
set fixed(value) {
Object.defineProperty(this, "fixed", {
value,
writable: false,
configurable: true,
});
}
doSomething() {
Object.defineProperty(this, "fixed", {
value: 24, //exact logic for changing the value is up to you
writable: false
});
}
}
const r = new ReadonlyFields();
r.fixed = 42;
console.log(r.fixed); //42;
r.fixed = 24;
console.log(r.fixed); //42;
r.doSomething();
console.log(r.fixed); //24;
If enforcing this behavior from a type system perspective is desired, simply make the property readonly:
class ReadonlyFields {
readonly fixed!: number;
setFixed(value: number) {
Object.defineProperty(this, "fixed", {
value,
writable: false,
configurable: true,
});
}
doSomething() {
Object.defineProperty(this, "fixed", {
value: 24, //exact logic for changing the value is up to you
writable: false,
});
}
}
const r = new ReadonlyFields();
r.setFixed(42); //OK
//@ts-expect-error
r.fixed = 24;
r.doSomething(); //OK