Description
Some properties in JavaScript are actually read-only, i.e. writes to them either fail silently or cause an exception. These should be modelable in TypeScript.
Previous attempts to design this have run into problems. A brief exploration:
interface Point {
x: number;
y: number;
}
interface ImmutablePoint {
readonly x: number;
readonly y: number;
}
var pt: ImmutablePoint = { x: 4, y: 5 }; // OK, can convert mutable to non-mutable
pt.x = 5; // Error, 'pt.x' is not a valid target of assignment
var pt2: Point = pt; // Error, cannot convert readonly 'x' to mutable 'x'
// Possibly bad behavior
var pt3: Point = { x: 1, y: 1 };
var pt4: ImmutablePoint = pt3; // OK
pt3.x = 5; // pt4.x is also changed?
// Really bad behavior
/** This function was written in TypeScript 1.0 **/
function magnitudeSquared(v: { x: number; y: number }) {
return v.x * v.x + v.y * v.y;
}
// Now try to use it with ImmutablePoint
console.log(magnitudeSquared(pt)); // Error, cannot use readonly object in non-readonly callPossible solutions?
- Allow bivariance of mutability: this is very unsound
- Something else clever? C++ did not do well with const contamination