ag-grid gridOptions.api undefined in angular 2

I am trying ag-grid in angular2 with typescript, for some
reasons I am not able to use the ag-grid APIs, getting undefined
error.,

here is the
code..,

import {
            AgRendererComponent } from 'ag-grid-ng2/main';
import { GridOptions, RowNode }
            from 'ag-grid/main';
import { GridOptionsWrapper } from
            'ag-grid/main';
import { GridApi } from
            'ag-grid/main';

public gridOptions:
            GridOptions;


constructor()
 {

            this.gridOptions = {};


            alert(this.gridOptions);
 alert(this.gridOptions.api); // *** getting
            undefined ***


 this.gridOptions =
            {

 columnDefs: this.columnDefs(),

            rowData: this.rowData,
 onSelectionChanged:
            this.onSelectionChanged,
 groupSelectsChildren: true,

            suppressRowClickSelection: true,

 rowSelection:
            'multiple',
 enableColResize: true,
 enableSorting: true,

            rowHeight:
            45}


}//constructor

Please
advise, Thanks

Updated with code
in comment
below

onGridReady()
            {
 console.log(this.gridOptions.api); // here it work


            this.selectedRows = this.gridOptions.api.getSelectedRows();

            console.log(this.selectedRows);
}

private
            testClick(event): void {
 try {
 console.log(this.gridOptions.api);
            // here gives error
 this.selectedRows =
            this.gridOptions.api.getSelectedRows();
 console.log(this.selectedRows);
            //getting error saying undefined

            }

}