Convert an array to a list of nested objects - Eloquent Javascript 4.3

Question

I am looking at this exercise from the book Eloquent Javascript, Chapter 4 - A List for quite some time and I\'m trying to understand how this particular function works:

Answer 1

I've tried to explain a little here. Let me know if something is unclear

function arrayToList(array) {
  // declare a empty variable to use is as a list
  let list = null;

  // Now iterate from the last element to the first. Example [10, 20]
  for (let i = array.length - 1; i >= 0; i--) {

    // iteration 1: i = 1 
    // we assign to list...
    list = {
        //the value of the current element
        value: array[i], // value = 20
        // the current list value before current assign
        rest: list // rest = null
    };
    // now it is assigned. list = {value: 20, rest: null}

    // ....

    // iteration 2: i = 0
    // we assign to list...
    list = {
        //the value of the current element
        value: array[i], // value = 10
        // the current list value before current assign
        rest: list // rest = {value: 20, rest: null}
    };
    // now it is assigned. list = {value: 10, rest: {value: 20, rest: null}}

  }
  return list;
}

Answer 2

TL;DR: In each iteration, we create a new object containing the object from last iteration.

In the line list = {value: array[i], rest: list};, the statement on the right of the = gets evaluated first. This means we create an object {value: array[i], rest: list}, containing the current values of array[i] and list. In the first iteration, list is null and array[i] is 20, so the object looks like this:

{value: 20, rest: null}

Only then do we assign this object to list.

In the next iteration, list isn’t null anymore, but {value: 20, rest: null}. So now, the object we create and assign to list looks like this:

{value: 10, rest: {value: 20, rest: null}}