cityList[index].forEach(function (val, index) { citySelect.add(new Option(val, index)); })

This is a code snippet in JavaScript that iterates through an array of cities and adds each city as an option to a select element in an HTML document. Here's a breakdown of the code:

cityList[index].forEach(function (val, index) {
  citySelect.add(new Option(val, index));
})

- `cityList` is an array of cities, likely defined elsewhere in the code.
- `index` is the index of the element in `cityList` that we want to iterate through.
- `forEach()` is a method that loops through each element in an array and executes a function for each element.
- The function passed to `forEach()` takes two parameters: `val`, which is the current element being iterated, and `index`, which is the index of that element in the array.
- `citySelect` is a reference to a select element in the HTML document, likely obtained using the `document.getElementById()` method or a similar method.
- `new Option(val, index)` creates a new `Option` object with a text value of `val` and a value attribute of `index`.
- `citySelect.add()` adds the `Option` object to the `citySelect` select element.

So this code essentially populates a select element with a list of cities. The `val` variable is used as the text of the option, while the `index` variable is used as the value attribute of the option. This can be useful for associating additional information with each city, such as a unique ID or a latitude/longitude pair.