array_unflatten.js

var arr = [
        {'id':1 ,'parentid' : 0},
        {'id':2 ,'parentid' : 1},
        {'id':3 ,'parentid' : 1},
        {'id':4 ,'parentid' : 2},
        {'id':5 ,'parentid' : 0},
        {'id':6 ,'parentid' : 0},
        {'id':7 ,'parentid' : 4}
];

unflatten = function( array, parent, tree ){
   
    tree = typeof tree !== 'undefined' ? tree : [];
    parent = typeof parent !== 'undefined' ? parent : { id: 0 };
        
    var children = _.filter( array, function(child){ return child.parentid == parent.id; });
    
    if( !_.isEmpty( children )  ){
        if( parent.id == 0 ){
           tree = children;   
        }else{
           parent['children'] = children
        }
        _.each( children, function( child ){ unflatten( array, child ) } );                    
    }
    
    return tree;
}

tree = unflatten( arr );

console.log(tree)

gistfile1.txt

  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
    },
    {
      id: 6,
      parent_id: null
    },
    {
      id: 7,
      parent_id: 6
    }
  ];

  const nest = (items, id = null, link = 'parent_id') =>
    items
      .filter((item) => item[link] === id)
      .map((item) => ({...item, children: nest(items, item.id)}));

  console.log(nest(comments));

gistfile2.txt

  var data = [
    {
      name: 'root',
      id: 1,
      parentLocation: null
    },
    {
      name: 'a2',
      parentLocation: {
        id: 1
      },
      id: 2
    },
    {
      name: 'a3',
      parentLocation: {
        id: 2
      },
      id: 3
    },
    {
      name: 'a4',
      parentLocation: {
        id: 1
      },
      id: 4
    },
    {
      name: 'a5',
      parentLocation: {
        id: 3
      },
      id: 5
    }
  ];

  const unflatten = (data) => {
    var idToNodeMap = {};
    var parentNode = [];
    var root = null;
    for (var i = 0; i < data.length; i++) {
      var datum = data[i];
      datum.children = [];
      idToNodeMap[datum.id] = datum;

      if (!datum.parentLocation) {
        root = datum;
      } else {
        parentNode = idToNodeMap[datum.parentLocation.id];
        parentNode.children.push(datum);
      }
    }

    return root;
  };

  console.log(unflatten(data));

gistfile3.txt

You can also use Map object, introduced in ES6.

let nodes = [
  { id: 1, pid: 0, name: "kpittu" },
  { id: 2, pid: 0, name: "news" },
  { id: 3, pid: 0, name: "menu" },
  { id: 4, pid: 3, name: "node" },
  { id: 5, pid: 4, name: "subnode" },
  { id: 6, pid: 1, name: "cace" }
];
  
  
  function toTree(arr) {
    let arrMap = new Map(arr.map((item) => [item.id, item]));
    let tree = [];

    for (let i = 0; i < arr.length; i++) {
      let item = arr[i];

      if (item.parentLocation) {
        let parentItem = arrMap.get(item.parentLocation.id);

        if (parentItem) {
          let {children} = parentItem;

          if (children) {
            parentItem.children.push(item);
          } else {
            parentItem.children = [item];
          }
        }
      } else {
        tree.push(item);
      }
    }

    return tree;
  }

  let tree = toTree(nodes);

  console.log(tree);