Transform flat array into tree view. Wrapped in Angular2 pipe

tree.pipe.ts

import { Pipe, PipeTransform } from '@angular/core';

@Pipe({ name: 'tree' })
export class TreePipe implements PipeTransform {
  transform(arr, args?: any[]) {
    return this.buildTree(arr)
  }

  buildTree(elements, tree = []) {    
    elements.forEach((element) => {
      if(!element.parent){
        element.children = this.getNestedChildren(elements, element._id)
        tree.push(element)
      }
    })
    return tree;
  }

  getNestedChildren(elements, parent) {
    let out = []
    elements.forEach((element) => {
      if(element.parent == parent){
        let children = this.getNestedChildren(elements, element._id)
        if(children.length > 0){ element.children = children; }        
        out.push(element)
      }
    })
    return out
  }
}

Expected input:
[ { id: 1 },{ id: 2 parent: 1 },{id: 3, parent: 1 }, {id: 4, parent: 2} ]