How does a closure work when there's only one parameter but two variables in it?

Question

Here's the function that I'm talking about:

function multiplier(factor) {
    return number => number * factor;
}

let twice = multiplier(2);
console.log(twice(5));

// output → 10

In this function shouldn't number be undefined since we're only giving one parameter. And that parameter gets multiplied and stored in number.. but anything that multiplies with undefined is NaN , right? Is the number = 1 here somehow?

Answer 1

Since multiplier function returning an arrow function with value of factor , it is a different syntax for following simple definition.

function twice(number) {
   // Value 2 is part of function definition as returned from multiplier() function
   return number * 2;
}

In console.log(twice(5)) you are passing parameter value 5, so it will return 5 * 2 .

Answer 2

function multiplier(factor) {
    return number => number * factor;
}
let twice = multiplier(2);

multiplier returns a function,so twice is like this:

twice = number => number * factor