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:

function arrayToList(array) {
  let list = null;
  for (let i = array.length - 1; i >= 0; i--) {
    list = {value: array[i], rest: list};
  }
  return list;
}

console.log( arrayToList([10, 20]));
// → {value: 10, rest: {value: 20, rest: null}}

Even after adding the list to the Watch window in the Debugger I can't clearly see the following:

1. How does exactly the statement from list = {value: array[i], rest: list}; get appended to the rest property of the nested object, during each iteration?

Obviously, the dot .rest or bracket ['rest'] notation are not being used, and it is not being explicitly specified that during every following iteration we are referring to the property of the object or its later nested objects, so how does it get appended to the property of each next nested object.

I am expecting for every iteration to overwrite the contents of the list object, but this is not the case.

1) Because it goes from back to front.

Luis felipe De jesus Munoz
– Luis felipe De jesus Munoz

2018-09-04 19:50:27 +00:00
Commented Sep 4, 2018 at 19:50 — Luis felipe De jesus Munoz
– Luis felipe De jesus Munoz, Commented Sep 4, 2018 at 19:50

Luis felipe De jesus Munoz · Accepted Answer · 2018-09-04 19:56:42Z

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;
}

Fii · Accepted Answer · 2018-09-04 20:01:18Z

1

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}}

answered Sep 4, 2018 at 20:01

Fii

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