How to call JavaScript functions from Typescript in Angular 5?

The given answer by Arun Raj R is perfect.

But for the angular 6+ project you need to take angular.json instead of angular-cli.json.

also if you want to pass parameter inside external function that come from js file than use just function name.

For Ex :

app.component.ts

import { Component, OnInit, ViewChild, ElementRef } from '@angular/core';
declare function myfunction: any; // just change here from arun answer.

@Component({
  selector: 'app-test',
  templateUrl: './test.component.html',
  styleUrls: ['./test.component.scss']
})
export class TestComponent implements OnInit {
    constructor() { }

    ngOnInit() { 
        myfunction('test1', 'test2');
    }
};

yourJs file :

function myfunction(params1, params2) {
    console.log('param1', params1);
    console.log('param2', params2);
}