Is there a way how you can get immutability in typescript of function parameters? (the same from java with final)
Example:
function add(a: number, b: number): number {
a = 5; // possible
return a + b;
}
And what I am searching for, example:
function add(a: ???, b: ???): number {
a = 5; // not possible because it would be immutable
return a + b;
}
I am searching this because of clearity if a function can modify my parameters.
If you're looking for immutability to prevent your function from causing side effects, you can use the Readonly<T> type.
interface Person {
name: string,
age: number,
}
function add_age(person: Readonly<Person>) {
person.age += 5; // not possible because it is immutable
}
add_age({ name: "Bob", age: 37 });
a, since that'd be limited to the scope of our function. What we care about is modifying a parameter object in place and have side effects. Tho it's true that it's not what OP asked for.It's a little hacky but you can enforce it like this
function add(...params: readonly [a: number, b: number]): number {
const [a, b] = params;
a = 5;
//^^^^^
//Cannot assign to 'a' because it is a constant.
return a + b;
}
a and b are now constants and therefore immutable.
Note however that although params is readonly that only prevents you from doing things like pushing or splicing the tuple array. It won't prevent you from reassigning the array altogether
params could be reassigned. But a and b are constants and therefore immutable at the JavaScript level. Trying to reassign either a or b will result in a runtime error - in addition to it being a TS type error.In addition to Aron's answer, you could also allow add to take 0..n arguments and make them readonly:
function add(...args: readonly number[]): number {
args[0] = 123; // not allowed!
return args.length === 0 ? 0
: args.length === 1 ? args[0]
: args.reduce((a, b) => a + b);
}
add() // 0
add(123) // 123
add(123, 456) // 579
add(123, 456, 789) // 1368
...again, it's hacky but it works.
