Angular 6 - Add new row in AG Grid

For insert a new row into ag-grid you shouldn't use the rowData directly it will create\override existing object and all states would be reset, and anyway, there is a method for it setRowData(rows)

But I'd recommend to use updateRowData(transaction):

updateRowData(transaction) Update row data into the grid. Pass a transaction object with lists for add, remove and update.

As example:

gridApi.updateRowData({add: newRows});

for angular:

set id for html - selector (#agGrid in this example):

<ag-grid-angular
  #agGrid 
  style="width: 650px; height: 500px;"
  class="ag-theme-balham"
  [rowData]="rowData"
  [columnDefs]="columnDefs"
>
</ag-grid-angular>

and then define the viewchild with this id, import AgGridAngular like shown below, then you can use the ag-grid api in Angular

import {Component, OnInit, ViewChild} from '@angular/core';
import { AgGridAngular } from 'ag-grid-angular';

@Component({
  selector: 'app-angular-handsometable',
  templateUrl: './angular-handsometable.component.html',
  styleUrls: ['./angular-handsometable.component.scss']
})
export class AngularHandsometableComponent implements OnInit {
  @ViewChild('agGrid') agGrid: AgGridAngular;
  columnDefs = [
    {headerName: 'Make', field: 'make', sortable: true, filter: true, editable: true },
    {headerName: 'Model', field: 'model', sortable: true, filter: true, editable: true },
    {headerName: 'Price', field: 'price', sortable: true, filter: true, editable: true }
  ];

  rowData = [
    { make: 'Toyota', model: 'Celica', price: 35000 },
    { make: 'Ford', model: 'Mondeo', price: 32000 },
    { make: 'Porsche', model: 'Boxter', price: 72000 },
    { make: 'Toyota', model: 'Celica', price: 35000 },
    { make: 'Ford', model: 'Mondeo', price: 32000 },
    { make: 'Porsche', model: 'Boxter', price: 72000 },
    { make: 'Toyota', model: 'Celica', price: 35000 },
    { make: 'Ford', model: 'Mondeo', price: 32000 },
    { make: 'Porsche', model: 'Boxter', price: 72000 },
    { make: 'Toyota', model: 'Celica', price: 35000 },
    { make: 'Ford', model: 'Mondeo', price: 32000 },
    { make: 'Porsche', model: 'Boxter', price: 72000 }
  ];
  constructor() { }

  ngOnInit() {

  }

  save() {
    console.log( 'Save', this.rowData );
  }

  addRow() {
    this.agGrid.api.updateRowData({
      add: [{ make: '', model: '', price: 0 }]
    });
  }

}