To load js file externally from assets
create service file add file to the assets and write path in array.  
import { Injectable } from "@angular/core";
    declare var document: any;
    @Injectable({
        providedIn:'root'
    })
    export class ScriptService {
      private scripts: any = {};
      constructor() {
        ScriptConstant.forEach((script: any) => {
          this.scripts[script.name] = {
            loaded: false,
            src: script.src
          };
        });
      }
      load(...scripts: string[]) {
        var promises: any[] = [];
        scripts.forEach(script => promises.push(this.loadScript(script)));
        return Promise.all(promises);
      }
      loadAll() {
        var promises: any[] = [];
        ScriptConstant.forEach(script => {
          // promises.push(delay(1000));
          promises.push(this.loadScript(script.name));
        });
        return Promise.all(promises);
      }
      loadScript(name: string) {
        return new Promise((resolve, reject) => {
          //resolve if already loaded
          if (this.scripts[name].loaded) {
            resolve({ script: name, loaded: true, status: "Already Loaded" });
          } else {
            //load script
            let script = document.createElement("script");
            script.type = "text/javascript";
            script.src = this.scripts[name].src;
            if (script.readyState) {
              //IE
              script.onreadystatechange = () => {
                if (
                  script.readyState === "loaded" ||
                  script.readyState === "complete"
                ) {
                  script.onreadystatechange = null;
                  this.scripts[name].loaded = true;
                  resolve({ script: name, loaded: true, status: "Loaded" });
                }
              };
            } else {
              //Others
              script.onload = () => {
                this.scripts[name].loaded = true;
                resolve({ script: name, loaded: true, status: "Loaded" });
              };
            }
            script.onerror = (error: any) =>
              resolve({ script: name, loaded: false, status: "Loaded" });
            document.getElementsByTagName("body")[0].appendChild(script);
          }
        });
      }
    }
    interface Scripts {
        name: string;
        src: string;
      }
      export const ScriptConstant: Scripts[] = [
        { name: "multislider", src: "assets/js/multislider.js" },
      ];
Inject this ScriptService wherever you need it and load js libs like this
this.script.load('filepicker').then(data => {
    console.log('script loaded ', data);
}).catch(error => console.log(error));