How can I use following interface?
interface Something {
(n: number): void,
description: string
}
I try writing following.
var sth: Something = {
description: "Description"
}
sth = (n: number) => {
console.log("Something");
}
or
var sth: Something = {
(n: number) => {
console.log("Something");
},
description: "Description"
}
But they both give errors.
You cannot directly create an object which is both a function and has other properties. You have to do it in two steps, first create a function, then assign it another prop, then assign the result to a variable of your interface:
interface Something {
(n: number): void;
description: string;
}
function fn(n: number) {}
fn.description = 'foo';
const h: Something = fn;
You can do it in one of few ways. Each fits slightly odd with functions with properties but all are viable, depending on the use case:
There is no syntax for creating a function with a property. You have to do both separately:
var sth: Something = ((n: number) => {
console.log(n + 2);
}) as Something;
sth.description = "Description";
The as Something assertion is needed here to tell the compiler to treat the function as Something, otherwise you cannot add the description.
Object.assign()This allows declaring everything at once:
var sth: Something = Object.assign(
(n: number) => {
console.log(n + 2);
},
{ description: "Description"}
);
Object.assign is typed to return any so the type declaration sth: Something makes sure it is treated as the correct object afterwards.
Object.defineProperty()Another way to create an object and add a property. It allows more control over the property, like making it non-writable, if needed.
var sth: Something = Object.defineProperty(
(n: number) => {
console.log(n + 2);
},
"description",
{ value: "Description"}
) as Something;
The as Something assertion is needed because Object.defineProperty() is typed to return the same as the first argument. So it returns (n: number) => void.
Object.defineProperties()Very similar to Object.defineProperty() but allows multiple properties to be configured at once.
var sth: Something = Object.defineProperties(
(n: number) => {
console.log(n + 2);
},
{
description: { value: "Description" }
}
) as Something;
