In the Vue.js source code (packages/reactivity/src/effects.ts), I found this:
export interface ReactiveEffectRunner<T = any> {
(): T
effect: ReactiveEffect
}
What does () mean in the code?
When you want to define a function with other properties you make use of this pattern.
() means that the object implementing this interface will be a function which can be called and does not need any params.
effect means it has another property called effect.
An example from the docs:
type DescribableFunction = {
description: string;
(someArg: number): boolean;
};
function doSomething(fn: DescribableFunction) {
console.log(fn.description + " returned " + fn(6));
}
Read about call signatures
It means the type is executable as a function.
for example:
declare const fn: ReactiveEffectRunner<{ abc: number }>
const result = fn() // { abc: number }
fn.effect // ReactiveEffect