ng_for_in-final.ts

import {
  Directive, TemplateRef, Input, ChangeDetectorRef, ViewContainerRef,
  IterableDiffers, OnChanges, SimpleChanges, SimpleChange
} from '@angular/core';
import { NgFor } from "@angular/common";
import { NgForRow } from "@angular/common/src/directives/ng_for";

@Directive({
  selector: '[ngFor][ngForIn]'
})
export class NgForIn extends NgFor implements OnChanges {

  @Input() ngForIn: any;

  constructor(viewContainer: ViewContainerRef,
              template: TemplateRef<NgForRow>,
              differs: IterableDiffers,
              cdr: ChangeDetectorRef) {

    super(viewContainer, template, differs, cdr);
  }

  ngOnChanges(changes: SimpleChanges): void {
    if ('ngForIn' in changes) {
      const value = this.ngForIn;

      const previousValue = changes['ngForIn'].previousValue;
      let newPreviousValue: Iterable<any>;
      let newValue: Iterable<any>;

      if (value instanceof Map) {
        // Map: iterate over an Array of its keys
        newValue = value && value.keys ? Array.from(value.keys()) : [];
        newPreviousValue = previousValue && previousValue.keys ? Array.from(previousValue.keys()) : [];
      }
      else if (value instanceof Object) {
        // Object: iterate over an Array of its keys
        // This also includes Arrays, in which case we will iterate over its indexes
        newValue = value && Object.keys(value);
        newPreviousValue = previousValue && Object.keys(previousValue);
      }
      else if ((value !== undefined) && (value !== null) && ((<any>value).length !== undefined)) {
        // For any other value with a "length" property, iterate over the indexes of an Array of that length
        newValue = Object.keys(Array.from({length: (<any>value).length}));
        newPreviousValue = Object.keys(Array.from({length: (<any>previousValue).length}));
      }
      else {
        throw new Error(`Error trying to get iterable properties from ${value}`);
      }

      this.ngForOf = newValue;

      changes['ngForOf'] =  new SimpleChange(newPreviousValue, newValue);
      super.ngOnChanges(changes);
    }
  }

}