Construct a hierarchy object from javascript objects

Here's my take on how to do this.

Create an object that will hold references to all the other objects, based on their key. This allows us to get a reference to each object by its key at the moment we need it as a parent of another element:

let register = {};

Create our output object:

let output = {};

Now let's go through each object in the array and add it to the structure:

// loop through the array
for (let el of elements) {
  // clone the element, so we don't modify the original array
  el = Object.assign({}, el);

  // insert the clone into the register
  register[el.id] = el;

  if (!el.parent) { // if no parent is set, add it to the top level
    output[el.id] = el;
  } else { // otherwise, add it as a child of whatever the parent is
    register[el.parent].children[el.id] = el;
  }

  // add a children property
  el.children = {};
  // remove the parent property
  delete el.parent;
}

Remember that objects are always stored by reference, so modifying an object in the register object also modifies it in the output object.

Below is a working example.

let input = [{
  id: 1,
  name: 'Jackson',
  parent: null
}, {
  id: 2,
  name: 'Jordan',
  parent: 1
}, {
  id: 3,
  name: 'Jefferson',
  parent: 1
}, {
  id: 4,
  name: 'Elvis',
  parent: 2
}, {
  id: 5,
  name: 'Sally',
  parent: null
}, {
  id: 6,
  name: 'Eric',
  parent: 4
}];

let register = {};

let output = {};

// loop through the array
for (let el of input) {
  // clone the element, so we don't modify the original array
  el = Object.assign({}, el);

  // insert the clone into the register
  register[el.id] = el;

  if (!el.parent) { // if no parent is set, add it to the top level
    output[el.id] = el;
  } else { // otherwise, add it as a child of whatever the parent is
    register[el.parent].children[el.id] = el;
  }

  // add a children property
  el.children = {};
  // remove the parent property
  delete el.parent;
}

console.log(JSON.stringify(output, undefined, 2));

Note that this will not work with circular references, or if the keys are out of order (i.e. the child appears before its parent).