JavaScript Making list from array

Question

I'm trying to make a list from arrays in JavaScript. I originally used dot notation to define the new values of each variable. However this produces what seems like an infinite list of the first value in the array.

{value: 10, rest: {value: 10, rest:{etc...}}}

The code given as a hint is the one commented out in the function below. When run, this gives the correct output (except assigning null to the last rest value which I haven't coded in yet).

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

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

What is the difference between using the two different methods?

"What is the difference between using the two different methods?" Which two methods? — frogatto
– frogatto, Commented Jul 16, 2016 at 17:27

Nina Scholz · Accepted Answer · 2016-07-16 17:59:15Z

3

You are making a circular reference with object, because you use the same object for assigning to object.rest.

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

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

The other example overwrites object with an assignment.

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

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

You could use a short ES6 version with Array#reduce for returning the rest object.

function toList(arr) {
    var result = {};
    arr.reduce((o, a) => (o.value = a, o.rest = {}), result);
    return result;
}

console.log(toList([2, 3, 5, 7, 11]));

edited Jul 16, 2016 at 17:59

answered Jul 16, 2016 at 17:27

Nina Scholz

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5 Comments

Morteza Tourani Over a year ago

Remove redundant result variable.

James Over a year ago

@NinaScholz I went over the basics of arrow functions. However, they give the general format of (parameters) => {statements}. I'm wondering why your statements are wrapped with () and separated with , instead of ;.

Nina Scholz Over a year ago

it's just to omit the return statement, needed with curly brackets. with parenthesis and the comma operator, the arrow function returns the last value, in this case {}, assigned to o.rest.

James Over a year ago

So by putting it in parenthesis, is it being treated as "one" statement that has several parts?

Nina Scholz Over a year ago

right. if no parenthesis, then the part o.rest = {} is treated as parameter of reduce.

Morteza Tourani · Accepted Answer · 2016-07-17 09:56:02Z

1

I think this could be another implementation for you linked list. You can simply change the link direction by replacing reduceRight with reduce.

function toList(arr) {
  return arr.reduceRight(function(rest, value) {
    return { value: value, rest: rest };
  }, null);
}

console.log(toList([1, 3]));

edited Jul 17, 2016 at 9:56

answered Jul 16, 2016 at 17:41

Morteza Tourani

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2 Comments

Nina Scholz Over a year ago

you could start with null.

Morteza Tourani Over a year ago

@NinaScholz yes that's the fun of reduce.

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