How to apply a function type to a function declaration-CodePudding

Given the following type:

type Add = (x:number, y:number) => number

I can declare a const of that type and define the rest of the function without having to specify the types explicitly. Typescript knows that x is a number and y is a number.

const add: Add = (x, y) => x   y;

I prefer to define functions using function rather than const however there doesn't appear to be a way to type the complete function declaration. It appears that you are forced to define the parameters and the return type separately. Is there any way to use the Add type on the declaration below so that I don't have to specify that x is a number and y is a number

function add(x, y) {
    return x   y;
}

without being forced to do something like

function add(x: Parameters<Add>[0], y: Parameters<Add>[1]): ReturnType<Add> {
    return x   y;
}

CodePudding user response：

You can't specify types for function declarations. You can only use Parameters and ReturnType. The only minor improvement would be to use destructuring for the parameters:

type Add = (x:number, y:number) => number
function add(...[x, y]: Parameters<Add>): ReturnType<Add> {
    return x   y;
}

Playground Link