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
    }
}