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  1. // A LIST
  2.  
  3. const arrayToList = (arr) => {
  4.   if (arr.length == 0) return;
  5.   let val = arr.pop();
  6.   return { value: val, rest: arrayToList(arr) }
  7. }
  8.  
  9. const listToArray = ({value, rest}, result = []) => {
  10.   result.push(value);
  11.   if (rest == null) return;
  12.   listToArray(rest, result);
  13.   return result;
  14. }
  15.  
  16. const prepend = (value, rest) => ({ value, rest });
  17.  
  18. const nth = ({value, rest}, position) => {
  19.   if (position == 0) return value;
  20.   return nth(rest, position - 1);
  21. }
  22.  
  23. console.log(arrayToList([10, 20]));
  24. // → {value: 10, rest: {value: 20, rest: null}}
  25. console.log(listToArray(arrayToList([10, 20, 30])));
  26. // → [10, 20, 30]
  27. console.log(prepend(10, prepend(20, null)));
  28. // → {value: 10, rest: {value: 20, rest: null}}
  29. console.log(nth(arrayToList([10, 20, 30]), 1));
  30. // → 20
  31.  
  32. // DEEP COMPARISON
  33.  
  34. const deepEqual = (a, b) => {
  35.   if (a === b) return true;
  36.  
  37.   if (a == null || typeof a != "object" ||
  38.       b == null || typeof b != "object") return false;
  39.  
  40.   const keysA = Object.keys(a), keysB = Object.keys(b);
  41.  
  42.   if (keysA.length != keysB.length) return false;
  43.  
  44.   for (let key of keysA) {
  45.     if (!keysB.includes(key) || !deepEqual(a[key], b[key])) return false;
  46.   }
  47.  
  48.   return true;
  49. }
  50.  
  51. let obj = {here: {is: "an"}, object: 2};
  52. console.log(deepEqual(obj, obj));
  53. // → true
  54. console.log(deepEqual(obj, {here: 1, object: 2}));
  55. // → false
  56. console.log(deepEqual(obj, {here: {is: "an"}, object: 2}));
  57. // → true
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