How to pass parameters from angular4 component to express API - node.js

Question

I am working for mean stack application. I am able to connect express api and angular component, but i want to pass parameters to the api service.
Please find the code below for clearer idea,
Component Code
constructor(private _dataService: DataService){
var parametervalue = "Monthly";
this._dataService.getexternalSourceDetailFiltered().subscribe((data) => {
this.source.load(data);
});}
DataService Code
import { Injectable } from '#angular/core';
import { Http } from '#angular/http';
import 'rxjs/add/operator/map';
#Injectable()
export class DataService {
result;
constructor(private _http: Http) { }
getStudents(){
return this._http.get('/external_sources').map(result =>
this.result = result.json().data);
}
getexternalSourceDetail(){
return this._http.get('/external_sources_details').map(result =>
this.result = result.json().data);
}
getexternalSourceDetailFiltered(){
return this._http.get('/external_sources_details').map(result =>
this.result = result.json().data);
}
}
Express API Code
router.get('/external_sources_details_filtered',(req,res) =>{
connection((db) => {
var intId = parseInt(0);
var query ={'frequency.Monthly':{$exists:true}};
var projection = {_id:0,sourceID:1,SourceName:1, Outstanding:1};
db.collection('external_sources').find(query).project(projection).
toArray().then((external_sources_details_filtered) => {
response.data = external_sources_details_filtered;
res.json(response);
})
})
})
How would i pass parametervalue from the component so that i can use it in express API to pass parameter to call mongodb using dynamic parameter

Answer 1

SOLUTION: Being totally new i searched around and found a solution:
i used URLSearchParams to set the parameter to pass through the express API.
Here is the the code for better understanding,
Component Code:
constructor(private _dataService: DataService){
var param = new URLSearchParams();
param.append('frequency','Monthly');
this._dataService.getexternalSourceDetailFiltered(param).subscribe((data) => {
this.source.load(data);
});
}
Data Service Code
getexternalSourceDetailFiltered(parameterValue:any ){
return this._http.get('/external_sources_details_filtered',
{
params:parameterValue}).map(result => this.result = result.json().data);
}
Express API js Code
router.get('/external_sources_details_filtered',(req,res) =>{
let parameterValue;
connection((db) => {
if(req.query.frequency != '')
{
parameterValue = String( 'frequency.'+ req.query.frequency);
}
else
{
parameterValue = String( 'frequency');
}
console.log(parameterValue);
var query = {[parameterValue] :{$exists:true}};
var projection = {_id:0,sourceID:1,SourceName:1, Outstanding:1};
db.collection('external_sources').find(query).project(projection).toArray().then((external_sources_details_filtered) => {
response.data = external_sources_details_filtered;
res.json(response);
})
})