Why no compiler error for this closure function?

Question

I am reading Marijn Haverbeke's excellent book, "Eloquent JavaScript". https://eloquentjavascript.net/

I do not understand this example on closures where number is not defined, yet there is no error.

Is number a function or a parameter?

There is nothing to indicate that number is passed in as parameter the second time.

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

let twice = multiplier(2);
console.log(twice(5));
// → 10

Answer 1

Understanding how closures work is hard enough but when the example you're looking at arbitrarily mixes function declarations with an arrow function - as this one does - if you don't understand how arrow functions work it makes it more difficult to understand.

Here's a slightly easier example that doesn't use an arrow function to show what's going on.

 // `multipler` takes a factor as an argument function multiplier(factor) { // It returns a function that - when it's called - // accepts a number return function (number) { // And the return from that function // is the factor * number return number * factor; } } // So we call `multipler` with a factor and assign the // function it returns to our `twice` variable let twice = multiplier(2); // We can then call the function assigned // to `twice` with a number, and the resulting // return from that function will be factor * number console.log(twice(5));

In terms of the example using that arrow function:

 // We pass in a factor to the `multipler` function multiplier(factor) { // We return a function that accepts a number // and returns factor * number // (I've added parentheses around the number // parameter to clearly show it) return (number) => number * factor; } // So we call `multipler` with a factor and assign the // function it returns to our `twice` variable let twice = multiplier(2); // We can then call the function assigned // to `twice` with a number, and the resulting // return from that function will be factor * number console.log(twice(5));