To define a function type, below example does not allow const
interface Square{
readonly color: string;
readonly area: number;
}
interface compareTo{
(const v1: Square, const v2: Square): number; // syntax error
}
How to compare v1 & v2 without changing the pointer of an object(Square) it is pointing to?
There's no way to declare this because it would have no effect on anything. const in TypeScript doesn't mean immutable -- it applies only to a binding, and a function parameter is not an externally-visible binding.
As another answer explains, const doesn't prevent objects from being modified in ES6, it only prevents reassignments.
In order to prevent parameter reassignments globally, TSLint no-parameter-reassignment rule can be used.
In order to prevent object modifications at runtime, Object.freeze should be used. This can be enforced at compilation time with Readonly mapped type. However, this won't have any effect if the types are compatible:
interface compareTo {
(v1: Readonly<Square>, v2: Readonly<Square>): number;
}
const foo: compareTo = (a: Square, b: Square) => {
a.area = 0;
return 1;
}
Generally, this isn't the responsibility of an interface to tell how function should internally work, it just describes its interface, so it would be:
interface compareTo {
(v1: Square, v2: Square): number;
}
const foo: compareTo = (a: Readonly<Square>, b: Readonly<Square>) => {
a.area = 0; // error
return 1;
}
But this will work if types are inferred from generic type, i.e. a type isn't specified in compareTo implementation:
interface compareTo<T = Readonly<Square>> {
(v1: T, v2: T): number;
}
const foo: compareTo = (a, b) => {
a.area = 0; // error
return 1;
}
v1 can be modified. We do this in Java, C. Anyways, let foo:compareTo = (a: Readonly<Square>, b: Readonly<Square>) => {a = {color: 'red', area: 20}; return 1;} allows modification
map, filter, sort aren't typed to prevent object modifications, even though it's rarely a good idea there.interface Square{readonly color: string; readonly area: number;} would suffice for not allowing a.area=0. I did not get the value-add of using Readonly<Square>readonly fields in Square and enforce them in compareTo only. If you didn't, it still remains unclear what purpose const is supposed to serve in your case.a = {color: 'white', area: 20}; in the function. const helps in such scenario for variables unlike parameters.
constor not inside function.v1andv2objects are only read but not written. here is a scenarioconstdoesn't prevent objects from being modified, as the answer explains. Please, provide an example that shows what you're trying to achieve/avoid.