JS递归function

Question

I defined a function let sum = a => b => b?(a+b):a;

If I do console.log(sum(1)(2)()); I get the output 3

But If I try console.log(sum(1)(2)(3)()); I get an error.

Why is that?

Answer 1

When you do

console.log(sum(1)(2)(3));

it will throw an error because you are trying to invoke the 3rd function; which you never created ( sum(1)(2) returns 3, which isn't a function).

Your function is a higher order function of two order. A function that returns another function. So the function call should be twice only; you mustn't call the 3rd function, as you did above.