打字稿返回值或咖喱函数

Question

Given the following Typescript code, I get an error

TS2349: Cannot invoke an expression whose type lacks a call signature. Type 'AddReturnType' has no compatible call signatures.

Why doesn't AddReturnType work with that call?

type AddReturnType = number | ((arg0: number) => number);
function add(x: number, y?: number) : AddReturnType {
    if (!y) {
        return (val) => val + y;
    }
    return x + y;
}

add(1)(2);

Answer 1

TypeScript can't determine whether you're returning a number or function, and only one of the two options is callable. Make the two function signatures separate:

function add(x: number): (number) => number;
function add(x: number, y: number): number;
function add(x, y?) {
    if (!y) {
        return (val) => val + y;
    }
    return x + y;
}

add(1)(2);

That said, shouldn't it be val + x ?