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JavaScript: Nest a given flat array of objects linked to one another recursively

JavaScript fundamental (ES6 Syntax): Exercise-90 with Solution

Write a JavaScript program to nest a given flat array of objects linked to one another recursively.

Note: Useful for nesting comments, such as the ones on reddit.com.

  • Use recursion.
  • Use Array.prototype.filter() to filter the items where the id matches the link.
  • Use Array.prototype.map() to map each item to a new object that has a children property which recursively nests the items based on which ones are children of the current item.
  • Omit the second argument, id, to default to null which indicates the object is not linked to another one (i.e. it is a top level object).
  • Omit the third argument, link, to use 'parent_id' as the default property which links the object to another one by its id.

Sample Solution:

JavaScript Code:

//#Source https://bit.ly/2neWfJ2 
const nest = (items, id = null, link = 'parent_id') =>
  items
    .filter(item => item[link] === id)
    .map(item => ({ ...item, children: nest(items, item.id) }));
// One top level comment
const comments = [
  { id: 1, parent_id: null },
  { id: 2, parent_id: 1 },
  { id: 3, parent_id: 1 },
  { id: 4, parent_id: 2 },
  { id: 5, parent_id: 4 }
];
const nestedComments = nest(comments); 
console.log(nestedComments);

Sample Output:

[{"id":1,"parent_id":null,"children":[{"id":2,"parent_id":1,"children":[{"id":4,"parent_id":2,"children":[{"id":5,"parent_id":4,"children":[]}]}]},{"id":3,"parent_id":1,"children":[]}]}]

Flowchart:

flowchart: Nest a given flat array of objects linked to one another recursively

Live Demo:

See the Pen javascript-fundamental-exercise-90 by w3resource (@w3resource) on CodePen.


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JavaScript: Tips of the Day

How to remove a specific item from an array?

Find the index of the array element you want to remove using indexOf, and then remove that index with splice.

The splice() method changes the contents of an array by removing existing elements and/or adding new elements.

const array = [2, 5, 9];

console.log(array);

const index = array.indexOf(5);
if (index > -1) {
  array.splice(index, 1);
}

// array = [2, 9]
console.log(array);

The second parameter of splice is the number of elements to remove. Note that splice modifies the array in place and returns a new array containing the elements that have been removed.

For the reason of completeness, here are functions. The first function removes only a single occurrence (i.e. removing the first match of 5 from [2,5,9,1,5,8,5]), while the second function removes all occurrences:

function removeItemOnce(arr, value) {
  var index = arr.indexOf(value);
  if (index > -1) {
    arr.splice(index, 1);
  }
  return arr;
}

function removeItemAll(arr, value) {
  var i = 0;
  while (i < arr.length) {
    if (arr[i] === value) {
      arr.splice(i, 1);
    } else {
      ++i;
    }
  }
  return arr;
}
//Usage
console.log(removeItemOnce([2,5,9,1,5,8,5], 5))
console.log(removeItemAll([2,5,9,1,5,8,5], 5))

Ref: https://bit.ly/2N9nKRp