Lifetime of local variables inside an Object Constructor Function

Question

In this example, taken from the W3Schools JS Tutorial, how is it possible for the variable n to still be used in the next function ?

// Home Made Iterable
function myNumbers() {
  let n = 0;
  return {
    next: function() {
      n += 10;
      return {value:n, done:false};
    }
  };
}

// Create Iterable
const n = myNumbers();
n.next(); // Returns 10
n.next(); // Returns 20
n.next(); // Returns 30

I'm coming from a C/C++ background and the lifetime of this variable makes no sense to me. It seems to behave like a global static variable but it should have been deleted when exiting myNumbers(), especially when declared with the let keyword. How can, then, the next method uses it without raising a ReferenceError and keeping its value updated ?

it's more like all variables are `std::shared_ptr`. So you can capture it no problem. — apple apple, Jul 13 '22 at 17:42

Mina · Answer 1 · 2022-07-13T17:39:54.190

1

Because of a closure feature of the JavaScript, keep the variable accessible even if the containing function died, whenever there is a child function still need to access it.

For more resources.

https://developer.mozilla.org/en-US/docs/Web/JavaScript/Closures

edited Jul 13 '22 at 17:39

answered Jul 13 '22 at 17:37

Mina

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Lifetime of local variables inside an Object Constructor Function

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