4.3 A List (Eloquent JavaScript Solutions)

4-3 Eloquent Javascript Solutions.js

function arrayToList(array) {
  let result = {};
  if (Array.isArray(array)) {
    let currListItem = result;
    for (let item of array) {
      let newListItem = {
      	value: item,
      	rest: null
      };
      if (typeof currListItem.rest === 'undefined') {
        result = newListItem;
      } else {
      	currListItem.rest = newListItem;
      }
      currListItem = newListItem;
    }
  }
  return result;
}

function listToArray(list) {
  let result = [];
  if (typeof list === 'undefined' || list.value === undefined || list.rest === undefined) {
    return result;
  } else {
    result.push(list.value);
    while (list.hasOwnProperty('rest') && list.rest !== null) {
      list = list.rest;
      if (list.hasOwnProperty('value')) {
      	result.push(list.value);
      }
    }
  }
  return result;
}

function prepend(element, list) {
  return {
    value: element,
    rest: list
  };
}

function nth(list, number) {
  return listToArray(list)[number];
}

function nthRecursive(list, number) {
  if (number === 0) {
    return list.value;
  } else if (list.rest === null) {
    return undefined;
  } else {
    return nthRecursive(list.rest, number-1);
  }
}


console.log(arrayToList());
console.log(arrayToList([10, 20]));
// → {value: 10, rest: {value: 20, rest: null}}
console.log(arrayToList([4, 3, 2, 1]));
// → {value: 4, rest: {value: 3, rest: {value: 2, rest: {value: 1, rest: null}}}}
console.log(listToArray());
// → []
console.log(listToArray(arrayToList([10, 20, 30])));
// → [10, 20, 30]
console.log(listToArray({value: 10, rest: {xxx: 20, yyy: null}}));
// → [10]
console.log(listToArray({value: 10, rest: {value: 20, yyy: null}}));
// → [10, 20]
console.log(listToArray({value: 10, rest: {xxx: 20, rest: null}}));
// → [10]
console.log(prepend(10, prepend(20, null)));
// → {value: 10, rest: {value: 20, rest: null}}
console.log(nth(arrayToList([10, 20, 30]), 1));
// → 20
console.log(nth(arrayToList([10, 20, 30]), 3));
// → undefined
console.log(nth(arrayToList([10, 20, 30]), -2));
// → undefined
console.log(nthRecursive(arrayToList([10, 20, 30]), 1));
// → 20
console.log(nthRecursive(arrayToList([10, 20, 30]), 3));
// → undefined
console.log(nthRecursive(arrayToList([10, 20, 30]), -2));
// → undefined

// now with the help of the hints...

function arrayToListWithHints(array) {
  let result = {};
  if (Array.isArray(array)) {
    let list = null;
    array = array.reverse();
    for (let item of array) {
      list = {
        value: item,
        rest: list
      };
    }
    result = list;
  }
  return result;
}

function listToArrayWithHints(list) {
  let result = [];
  if (typeof list === 'undefined' || list.value === undefined || list.rest === undefined) {
    return result;
  } else {
    for (let node = list; node; node = node.rest) {
      if (node.hasOwnProperty('value')) {
        result.push(node.value);
      }
    }
  }
  return result;
}

console.log('\nWith Hints:');
console.log(arrayToListWithHints());
console.log(arrayToListWithHints([10, 20]));
// → {value: 10, rest: {value: 20, rest: null}}
console.log(arrayToListWithHints([4, 3, 2, 1]));
// → {value: 4, rest: {value: 3, rest: {value: 2, rest: {value: 1, rest: null}}}}
console.log(listToArrayWithHints());
// → []
console.log(listToArrayWithHints(arrayToListWithHints([10, 20, 30])));
// → [10, 20, 30]
console.log(listToArrayWithHints({value: 10, rest: {xxx: 20, yyy: null}}));
// → [10]
console.log(listToArrayWithHints({value: 10, rest: {value: 20, yyy: null}}));
// → [10, 20]
console.log(listToArrayWithHints({value: 10, rest: {xxx: 20, rest: null}}));
// → [10]
console.log(nth(arrayToListWithHints([10, 20, 30]), 1));
// → 20
console.log(nth(arrayToListWithHints([10, 20, 30]), 3));
// → undefined
console.log(nth(arrayToListWithHints([10, 20, 30]), -2));
// → undefined

Hey @PiotrNap. Glad it helped you.
The reason that the first undefined is in single quotes is because typeof returns a string value so the comparison has to be a string as well.

Alright, that makes sense ! Ty for answering :)

Hello Jonurry, I'm a newbie in js so I have some questions about your solutions. I hope you can help me.

In arrayToList function, I don't know how the value of "result" changes in the loop. In your loop, the "result" only appears in the if (typeof currListItem.rest === 'undefined') but when I try testing it like this :

if (typeof currListItem.rest === 'undefined') {
result = newListItem;
console.log(true)
} else {
currListItem.rest = newListItem;
console.log(false)
}

The if (typeof currListItem.rest === 'undefined') only works in the first time, so I don't know how the "result" can change its value.