How do I pass the value (not the reference) of a JS variable to a function?

Question

Here is a simplified version of something I'm trying to run:

for (var i = 0; i < results.length; i++) {
    marker = results[i];
    google.maps.event.addListener(marker, 'click', function() { 
        change_selection(i);
    }); 
}

but I'm finding that every listener uses the value of results.length (the value when the for loop terminates). How can I add listeners such that each uses the value of i at the time I add it, rather than the reference to i?

Answer 1

In modern browsers, you can use the let or const keywords to create a block-scoped variable:

for (let i = 0; i < results.length; i++) {
  let marker = results[i];
  google.maps.event.addListener(marker, 'click', () => change_selection(i));
}

---

In older browsers, you need to create a separate scope that saves the variable in its current state by passing it as a function parameter:

for (var i = 0; i < results.length; i++) {
  (function (i) {
    marker = results[i];
    google.maps.event.addListener(marker, 'click', function() { 
      change_selection(i);
    }); 
  })(i);
}

By creating an anonymous function and calling it with the variable as the first argument, you're passing-by-value to the function and creating a closure.